Getting back to Revelation 1:5-6 for a bit, here's what it says according to the KJV.
Revelation 1:5 And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
6 And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen.
Verse 6 states..And hath made us kings and priests unto God and his Father.
Revelation 5:9 And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
10 And hast made us unto our God kings and priests: and we shall reign on the earth.
The first thing to keep in mind here, this scene would literally be in heaven. So while in heaven they are singing a new song. Now what does it say in this new song? And hast made us unto our God kings and priests: and we shall reign on the earth. So how exactly are those singing the new song in heaven supposed to reign on the earth as kings and priests, the fact they're now in heaven? Isn't it then logical that they're meaning when they return to the earth with Jesus, that's when they will be reigning as priests and kings? Something else to keep in mind here, if both the physically alive on the earth and those that have since passed on to heaven are to be reigning as priests with Christ a thousand years, then it looks like those in heaven are being cut out of their part in this reign, since Revelation 5:10 clearly shows this reigning takes place on earth, not in heaven.