I don't see problems with it all being in one bank? Perhaps if your computer doesn't run well - mine isn't fantastic, and though sometimes I get lag from loading some banks I think it's worth it to have it all together. But the most efficient way (in my opinion) of packaging multipitch is to have this layout
Voicebank Folder
- Main Pitch samples
- Main Pitch oto
> >Folder of Extra Pitch #1
- Extra Pitch samples
- Extra Pitch oto
so on and so forth
That way people just download one bank and it's all there. The otos all compile when you open the voicebank in UTAU, so don't worry about that.
And as a side note, you don't have to do arrows or plus/minuses. If you do more than two extra pitches that no longer works (and I just prefer this overall) - you can use letters and numbers. If your high pitch is recorded at C4 you figure out, rename the folder C4 and have the alias be C4. I find that easier because you have to copy/paste arrows (plus/minus is exempt from that) and it works beyond two extra pitches.
Again, personal preferences. If you find you like the other things go with those.