Great question. I have always played at stakes that my bankroll and skill level allowed, but you make a good point: What stakes serve as the "donkey filter"? I find that for Limit, the $10/$20 game filters out all but the rich donkeys, and they are - of course - ALWAYS welcome .

No Limit: There is no real filter here and I have seen absolutely attrocious plays at low and high stakes, due to the popularity of poker.

Pot Limit Omaha Hi: Live games generally have high limits, but as the popularity of this game increases, expect casinos to drop the blinds. Online there seems to be a very sharp line at the PL Omaha Hi $1/$2 level, above which donkeys dare not tread.

Pot Limit Omaha HiLo for some reason rarely gets a lot of total crazies in.

When I am playing to improve my game, I make sure I play as high as I can, to ensure a minimum number of luckboxes, but then again, I tend to be loose in my approach to the game, and one man's maniac is another's loose play.