The general rule which I've heard is you "don't tip the owner." If you are setting your own rates, it fits the same pattern. Not tipping is appropriate in this case, unless you feel like the rate was significantly under-priced for the quality of work performed. In cases like that, I've actually told someone, "for the work you're doing I'm paying you $50 not $35 per hour -- keep it up!" In other words, give them a raise, not a tip.