The thing is, you haven't offered any product or service for free that could incentive users to tip you.
I definitely think tipping is a good thing, and I try to tip whenever someone offers a free service that is useful and I use a lot. I don't know about spreading the word about BTC, because if someone accepts tips with BTC, they probably know about bitcoin beforehand. On the contrary, I think bitcoin tipping is very useful to reward people for offering a free service.
Though tipping is a good thing and i also feel it is worth doing if someone does something appreciable. But now a days we can hardly find someone tipping a person for doing something just because of the price of bitcoin, people even try to save the smallest parts of their possession so that they can earn more profit in coming time.