Only possible answer is "it all depends". The only absolute rule is to fly higher than the terrain. (Though I admit the first time ATC asked me if I had "terrain in sight" I started anxiously checking all the railway tracks in view).
For local puttering about I generally fly around 2-4000 feet. For typical xc flying I like to be around 7-10000 - gives me plenty of room if the fan stops, and a bit more TAS.I'll go higher or lower because of winds. I've flown up at FL200 a couple of times on very long flights to take advantage of winds.
Of course it all depends on airspace, too. Flying around San Francisco, I generally stay below 1500 so I can avoid talking to Norcal. (Why I would I want to do that? - because SFO tower will often give you a transition when you ask them directly, even though Norcal is refusing them).
In the heli I normally fly 1000-1500 AGL. Altitude is not really on your side, and it keeps you out of the way of the fixed wing stuff. And the view is wonderful.