Tipping a complete stranger with Bitcoins could piss them off -- (what is this crap?). Offering a tip in Bitcoin *or* fiat puts the receiver in a much better position -- (keep track of how often they choose Bitcoin vs. fiat and report back here). *If* you can/will see the receiver again in the near future then offering to replace the Bitcoin tip with fiat at that time will make them feel good.
I think thats not the point here. I'm sure you can find pissed off people even you give them gold. Let them be pissed of. Its primary to promote bitcoin, if they are happy with it is up to them.