In a way so do I, but I don't think of it as tipping as such, I just find it easier than rather waiting for them to faff around looking for change, I round up to the nearest £
With the exception of restaurants where, if the service has been exceptionally good, I tend to use the old 10% rule.
Tip a postman? Why exactly? They are paid to put my letters through my letterbox, which they do. Why would a postman deserve a tip for that? I tip people that do something extra, like a cabbie that helps me carry shopping bags to my door for example.