So when you join a faction, are all the rewards given to you based on the level when you joined up? Or based on the level you are when you completed the quest??

In a nut-shell.. I want to get into TDB Sanctuary because they have skill books down there that I need. But I dont want to join them and get lvl 20 rewards. You get the best rewards at lvl 35+. So do I have to wait and join at lvl 35 just to get the skill books?? Or can I join, not do any missions, get the skill books and leave. If I join at lvl 20 and come back at lvl 35 to do quests, will the rewards be lvl 20 rewards or lvl 35 rewards???

Thanks..

The items level is determined by the level you are in when you get them, so you can join the DB, wait till you're level 35, and do the quests after that.

Exception is Blade of Woe that you get during the DB initiation, it's level is determined by the level you are in at the beginning of "Honor thy Mother" quest, so again the above will work to get the best version of it.

The level of your quest rewards is determined as ToJKa says.

Most quest reward items in the game max out at either level 25 or 30. There are no quest reward items that max out above level 30.

All quest reward items for the DB questline max out at 30 except the following rewards, which are unleveled: Scales of Pitiless Justice, Night Mother's Blessing, Shadowmere.