Below CAS, UK Class G, I fly at funny numbers e.g. 2300 2700 3300 3700 4300 etc. Never below 2000ft (too much traffic).
CAS permitting I fly VMC on top (sunshine and nobody about), or about 5000ft.
On IFR/Eurocontrol flights I file for FL140 and stop climb at FL100 if the wx is clear, otherwise I climb to VMC on top, to FL190 if necessary (use
various means to check tops before going). Oxygen is always carried; no oxygen = no go.