First off, Stiler, you're the one who's being dumb. You said you gave the dude a handful of change? Did you think he actually counted it before he said thanks? He didn't. He probably assumed you counted out a little extra for him, said thanks, put it in his pocket, and ran off back to the shop or to the next delivery. The change in his pocket probably didn't get counted until the end of his shift, where it is lumped in with all the change from all the other poor fools who pay with coin. So, while paying for a $6 pizza in coin doesn't exactly leave a positive impression, you probably got lucky on this one. He knows you didn't tip well, but he doesn't know for sure that you stiffed him.
As far as bad things happening to your food, honestly, I don't care enough to spit in anyone's pizza, I'm in too much of a hurry. I've never done it, and I've never even heard of it happening where I work. (One thing I never do is give someone a reason to complain.)
If you stiff all the time, and I've got more than one 'za to deliver, and the route that I must take permits (one ways, dead ends, etc.), I will simply deliver your pie last. It's pretty bad for the stiffer if I've got to go downtown to a hotel before you get your pie. That can add a good 10-15 minutes to your wait. If it's a single (only one pie to deliver when I leave the store), I'll usually hurry just to beat the other drivers back in order to get the next run before they do. When it comes down to it, he who takes the most deliveries takes home the most money. Sometimes I'm up by 10 runs on some guys in the same night, which to me with my average (including mileage, which is paid in cash every night), is generally at least $30. The game of getting more runs is all about being smart about the roads and being quick while I'm out of the car. The game of getting a better tip is about being polite and professional, sometimes funny, and looking sharp and dressing well. And yeah, some people are cheap, but life goes on.