If you take care of Charlie Sheen's feet, he'll take care of you.

Our spa spies tell us Sheen dropped by to get his little piggies a pedicure -- and when all was said and done he dropped a $100 tip on a $50 bill. We're told he was "one of the nicest clients ever."

Considering he reportedly makes $800,000 an episode for "Two and Half Men" ... he's probably one of the richest too.