Practical AGL Limit  

I once claimed to 11,000 feet because reported winds ended up giving me 205 kts over the ground on a 600 nm repositioning flight, totally worth it. 

trouble is given the possibility of any emergency it's a very long 10-15 minutes back to the surface...

above 3k your bird strike risk is markedly reduced.  

flying in mountains... generally add 2k feet above highest elevations to avoid majo of mechanical turbulence.

when in doubt asses risks and Benifits and chose then evaluate... when in doubt just don't fly!