Perhaps I am a bit different, since I work for myself... I charge what I feel the work is worth.
From my perspective, coffee and a sandwich are an awesome tip. If you want to throw $20 our way, that's great too.. However, it is certainly not expected, nor is it necessary.
If you have hired guys who work for a third party, that may be different. It may be nice to flip them a little extra, but just understand that they MAY not be performing the highest of quality work. Otherwise, they'd be working for themselves.