Since I didn't see anyone else mention it...
Every boss after (and including) the floor 30 boss give guaranteed upgrades. I've only seen +1/+1 so far, so not sure if the upgrades can flare on bosses.
At any rate...
One you start making runs for floor 100, just buy a grip every time you get to 99/99. This will drop you to 89/89, which isn't going to make that much of an impact, especially if you are starting at floor 21. Between floors 21-90 you will fight 7 bosses that are guaranteed to give at least a +1/+1 upgrade... so you are guaranteed to be at least 96/96 when you start floor 91 from boss upgrades alone.
Also also...
Spoilers...
There is a "secret" mount that you can get after clearing HoH floor 100 a minimum of four times... so if you are going for that there is plenty of time to get grips.