A friend of mine told me not to worry about my opponent's foot-faults unless they were giving him an advantage.
To me, already being inside the court when striking the ball on serve is an advantage. Sharper angles, less time to react, not to mention the fact that they have a head start in getting to a drop shot.
I play be the rules, and figure my opponent should as well. If I nick a ball, I call it out. If there is a double bounce, I inform them. If I catch a ball in the air, I concede the point. If I touch the net before the ball bounces twice on their side, I announce that it's their point. Whether they saw it or not (which has happened to me on a few occasions).
I work hard at improving my serve and not foot-faulting. Why should I have to abide by a rule that they choose to ignore, or are unaware that they are violating?
I think they should be made aware of their infraction so they can correct it. I played one guy that foot-faulted every time he served to the add court because he stepped way to the right and crossed the center mark (and yes, I would say flagrantly) due to his extreme right moving ball toss.
I won the match, and afterwards, I took him aside and politely let him know about the issue. I told him he was not called on it during the match, but he needed to correct the problem before someone did start calling him on it and it cost him a point, a game, or a match.
He was aware that he was moving across the center line, but didn't know it was illegal since he started completely on the correct side. He only understood that he could not cross the baseline before striking the ball.
He, in turn, politely thanked me for bringing it to his attention in what he considered an honorable way, and left with no hard feelings that I am aware of. The next time I played him, the foot-faulting was gone and he actually served quite a bit better.
Actually calling foot-faults during a match is hard to do in singles unless it is flagrant, but if you see it, you should call it.
I play another gentleman on occasion that hits a very hard first serve, and a pretty darn good second as well. I wasn't paying attention to his feet because I was worried about returning his bullets. I video tape my matches and was shocked when I watched the match and saw that every time he served (first and second), his heal was the only part of his right foot that was not already totally inside the court. By this, I mean his heel was touching the inside of the baseline.
The next time I play him, I plan to watch his feet on the first few points and call the foot-faults right off the bat (warning first time, then points, correct?). I don't want to wait until I am behind an have it appear as gamesmanship. I just want him to play by the same rules. I don't mind losing to a better player, but I do want play to be correct on both sides of the net.
If he denies the problem, or refuses to score correctly, I plan to get the league director to come out and verify.
Evil doers ... you face The Tick!