Tipping upfront is proper, polite and I would say necessessary when you're in a formal environment where everyone knows what to expect and no one is out to screw, um, er, ...Take Advantage of anyone.
Though I'm new to the scene, the Passaic AMPs fit that description. I can not imagine not giving the standard tip up front, and even more if the situation dictates.
When with an unknown independent, I've found a flash of cash to be safer. A couple of times they've taken the upfront tip and then saved themselves the wear and tear of anything more than light service. But that's only for when you're really slumming it. If you slum it
A third technique has worked for me when my concern is that the AMP is generally legit. Then, I take out the tip and put it on top of my wallet on a table.
This opens up a quid pro quo, and I get to decide if my quid and my id are satisfied.
Above all, though, be fair. Be nice. Its supposed a fun activity!