When touring, typically in the 7500 - 9500 band, obviously at hemispheric levels. Both the normally aspirated engine of the a/c I mostly fly (C172RGs) as well as - importantly! - Mrs. 172driver are happy at these altitudes. An added advantage is that it usually gets you above the 'bimblers', there's not much traffic up there, but still plenty to look at on the ground. That said, I think the highest I've been in this type was around 13k for a short while.
In other a/c and if O2 available, then higher, the low teens are a nice place to be (well, at least in aviation
).
If on a bimble, well, anything goes (within legal limits, of course!)
although I have to say I prefer to be up high(ish) in general.
Interesting observation by Englishal (Adam, I guess you refer to his post and not Silvair's). Never had that problem, but can see how a low wing might give some idea of a 'floor' that a high wing doesn't.