Tipping: Is it good for the soul, or the wallet?

My husband and I are both outrageously generous tippers in a city where (many have cried) the tip jar is out of control. At my favorite local coffeeshop -- ok, actually at every coffeeshop in Portland -- I always tip a dollar unless I barely have enough for coffee, or my tab is over $10. My husband can be worse, tipping $3 on a $12 coffee-and-pastries tab. While we're not sure if the labor is worth it, the coffee definitely IS. Elsewhere I've tipped generously (can you be reluctant and generous, at once?) despite onerous requirements of the customer, what with large signs requesting that I bus my own dishes; often-empty water pitchers/creamer/etc.; and the occasional snotty barista. We all know that tipping less than 20% has become insulting.

Part of it is my experience in food service, but also, I persist in believing that tipping is just another way of spreading the karma; that where much good is given, much good will be returned. (I also figure that my consistent friendliness to the bus drivers is the reason I always find my left-on-the-bus belongings returned to me.) So what if I'm spending $2.75 for a $1.75 coffee? It's ridiculously awesome coffee, after all, and maybe the universe will reward me with something in return; maybe I'll get a good bonus at the end of the year; maybe I'll get people sending me Paypal contributions just because. I guess it's along the same lines as the concept that tithing will bring you wealth; just a little more secular and karmic.

Is there anything to this? Do you tip generously just in case the universe is watching? Or do you do it to make sure you, too, get your hallway vacuumed carefully? Do you tip way more at the holidays, or do you share your generosity equally throughout the year? Is tipping good for service, the soul, or the wallet; or all of the above?