Everyone, no matter who they are, will have to stand before the judgement seat of God, but God judges no one. Jesus will do the judging:
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For the Father judges no one at all, but he has entrusted all the judging to the Son."John 5:22.
Those called to heaven will be judged either before the end of this system of things, or before they die, as was the case of Stephen. He obviously had received the final sealing of approval. When that happened we have no idea. If it was when he was about to be stoned and given the vision in heaven, or before.
Often we apply the scripture of 2 Peter 3:9 to the world in general, and to the entire Christian congregation. But actually it is specifically talking to the anointed Christian congregation where it says:
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Jehovah is not slow concerning his promise, as some people consider slowness, but he is patient with you because he does not desire anyone to be destroyed but desires all to attain to repentance."-2 Peter 3:9.
It is obvious that he is referring to the anointed because he said that "Jehovah...is patient with
you" writing to the anointed Christian congregation (see verse 1). That this is referring to our time is evident, because we are in the time of the end. That is also consistent with what Revelation 7 tells us. There it says that God has the angels holding back the four winds of destruction until all the chosen have received the seal of God:
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And I saw another angel ascending from the sunrise, having a seal of the living God; and he called with a loud voice to the four angels to whom it was granted to harm the earth and the sea, saying: “Do not harm the earth or the sea or the trees, until after we have sealed the slaves of our God in their foreheads.”-Revelation 7:2-3.
I personally have known several of the anointed who have been disfellowshipped or strongly disciplined for works of the flesh that they need to receive full repentance from. Jehovah knows each and every one of his servants, and his patience with the anointed Christian congregation also means their salvation, and not only theirs, but that of all who avail themselves of his "year of goodwill" (Isaiah 61:1-2):
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Therefore, beloved ones, since you are awaiting these things, do your utmost to be found finally by him spotless and unblemished and in peace. Furthermore, consider the patience of our Lord as salvation."-2 Peter 3:14-15.
As Hockeycowboy pointed out the members of the Christian congregation who have an earthly hope will receive a judgement when Jesus arrives in his kingdom glory in connection with his holy ones to be glorified (2 Thessalonians 1:9-10). At that time he will separate the people as sheep from goats. The scripture says that Jesus will sit down on his throne to judge at that time:
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When the Son of man comes in his glory, and all the angels with him, then he will sit down on his glorious throne. All the nations will be gathered before him, and he will separate people one from another, just as a shepherd separates the sheep from the goats. And he will put the sheep on his right hand, but the goats on his left."-Matthew 25:31-33.
The goats will be cast into everlasting destruction. The sheep will inherit the kingdom blessings in God's new world on earth.
It will not be until the end of the thousand years of Christ that all of the dead who have been raised back to life, and all the faithful who pass the final test will receive the judgement of everlasting life according to Revelation 20:5:
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The rest of the dead did not come to life until the 1,000 years were ended."
Everyone therefore will stand before the judgement seat of God, while he will do no judging directly he has given that authority to his son. Jesus Christ will judge everyone. Now as was mentioned, when Stephen received that judgement, which obviously was favorable, we do not know when that was, so we can not say it was when he received that vision. I personally think he received his final sealing before then. Paul, before he was put to death, told us he had received the approbation in heaven that he had run the fine fight to the end:
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I have fought the fine fight, I have run the race to the finish, I have observed the faith. From this time on, there is reserved for me the crown of righteousness, which the Lord, the righteous judge, will give me as a reward in that day, yet not to me only, but also to all those who have loved his manifestation."-2 Timothy 4:7-8.
There is another interesting point about Stephen I like to point out to my Christian brothers and sisters in the faith that imagine that a person has to have been anointed for decades to receive the crown of life. Stephen, the first recorded Christian martyr in Holy Scriptures, died shortly after the Christian congregation was formed, around the year 33-34 C. E. The first Christians to be anointed were on Pentecost of 33 C. E. So he was only anointed for at most a year or so, if he was part of that group.
The same may happen during this time. Jehovah may decide according to his good will to chose the final ones of the 144,000 not very long before the end comes. And they also may die a violent death as did Stephen:
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And a white robe was given to each of them, and they were told to rest a little while longer, until the number was filled of their fellow slaves and their brothers who were about to be killed as they had been."-Revelation 6:11.
The most recent JW Broadcasting in the first part the brother giving the talk relates Revelation 6 with Matthew 24, Mark 13, and Luke 21.
https://www.jw.org/en/library/videos/#/en/categories/VODStudio
See minute 1:10.