Pardon if this is a FAQ, but what's the latest I should wait, in terms of level, before starting an end-game quest sequence? I want to avoid playing any part of the end-game with my level capped, i.e., with XP going to waste, so starting For The Republic Part 2 at level 29 is obviously bad. I also want to avoid starting too soon so that I end the game with a lot of levels yet to earn.

I plan to save right after setting up at least two possible end-games (NCR and House), if not three (Yes-Man) with my current character, so I can play through an end-game or two, and then go back to that save to play out any interesting side-quests that are left, leaving the end-game unresolved.

I'm currently at Level 22 on Normal on my first run through, and given my rate of leveling up, I think I should be starting a sequence pretty soon. Can I wait until about Level 26, or would that be too late?

Well you can always choose to do something else for a while. It's not a necessity once you start, you can't deviate. A warning before the final mission even gives you the option to do some other things first. I'd say you can start now and always try something else before you finish.