FL100-FL120 preferred, this is a very efficient altitude for our plane. We do have (portable) oxygen to go higher, but rarely need to.
Strong winds, clouds, CAS and routing restrictions might interfere.
Someday (after getting oxygen masks instead of cannulas) we're going to try getting up to the plane's ceiling, just for the fun of having checked that she can really get there, which is pretty high for a non-turbo.
Last summer, I did a flight at low altitude (between 600-750') in marginal weather (between 2-4 miles vis and rain) over unfamiliar ground.... this would have been extremely uncomfortable at our usual economic cruise speed, let alone at 75%... so instead of pretending to be a fighter on a strafing run I throttled way back and lowered the wheels and pretended to be a microlight, not comfortable either but a lot safer. The slower you are the lower you can still navigate safely in less than ideal weather circumstances.