I always fligh as high as possibly, where practical. My record to date has been FL210 over Norwich
For me the limiting factor is airspace in the UK. In the US, its not uncommon to fly at 10000-12500' in a SEP, though this is normally due to terrain. 8000' in a SEP is a comfortable altitude.
Advantages include not hitting everyone below 2000', Evo at 2200, Tall_guy_in_a_152 at 2250, gliders, and its easy to avoid certain airspaces, like MATZ or ATZ's. Shame Class D doesn't stop at 5000' and overlaying Class A doesn't start till 10,000', then we'd have loads of room to play around in
Cheers
EA