I tip "service workers" whether it's called for or not, mainly because I've been there, and it does make a difference. Even if it doesn't make a significant difference to their bottom line, it makes people feel appreciated and is just a nice thing to do. If that person reciprocates with better service in the future, as sometimes happens, it's money well spent. If not, you're out $5-10 that would have just gone to the commemorative Mao trinket industry anyway.
But yeah, I wouldn't do it out of any feeling of obligation, or out of some weird-ass desire to bond with the help. I just do it to help somebody else who's where I was at not too long ago. It's not like a moral imperative or anything.