When we negotiate with people on jobs, we almost always pay upon completion, for the reasons Lee has described. If they hassle us, we basically tell them "Ask around about us. You'll find that everyone will tell you that we pay on time." And then we're clear about what the expectations are so there's no confusion.

We have gotten mildly burned on projects where, for example, the job was 90% complete and the person asked for money then never finished debugging, or else they sold us a product that wasn't mature and didn't work well... but even in those cases, we've pretty much learned from it.

I think there are some people who are total scammers, but most are like so many others in the industry... not particularly hard workers who don't intend to scam anyone but are distracted by more "fun" pursuits once they have their money.