I usually play with a 12" tom up.
For the stuff I play, I don't think I'd want to go much smaller than that, because I prefer to use a 16" floor tom in a small kit, so I don't watch such a pitch difference between toms.
True. No rules. But back in the day, if a company offered a five-piece kit, say 12/13/16/22, the four piece would've been 13/16/22. But these days a lot of companies offer 12/16/22 as a configuration now instead.