If you're going to add a credit button, then you still have it on "free play", you just have to press a button to get it. It still "devalues" winning a credit, because if you want to play and you didn't earn the credit in-game, you're still going to press the credit button anyway.
I hate credit buttons, as there are ways to have access to home play without them. I agree with Isochronic_Frost 100%, credit buttons are hacks and usually end up putting a hole in the coin door or cabinet, and look like crap. Like he suggested, if you must, make sure it's not a permanent modification that can be reversed.
I'm not familiar with P2K, but can you not add credits via the operator buttons, like earlier Williams WPC games or modern Sterns?