Perhaps you should quote the part of the DMG in the preface that says that it's the DM's game, and that he makes the final call on all rules. I'm afraid I don't have an exact quote, so you will have to look that one up.
For my experience, I'll take time to ask a DM if a rules change is meant in a circumstance, but if he says something is final, then I do not pursue it any further. If it's worth more than ten or fifteen seconds of game time, it's too disruptive IMO, and I leave it alone.
Recent example: This past weekend's Eberron game saw one of my players a little off-put when I allowed the Warforged Fighter PC in our group an attack I technically shouldn't have. Big flying thing passes by only ten feet over his head; he stands 6 foot five, AND has a greatsword, so he asked if he could leap and attack once as it passed by. In a spirit of fun, I told him he could, but gave him a -4 penalty to the attack. One of the other players thought I had just set a precedent for allowing reach attacks at a -4 penalty. In my opinion, I started a precedent to take a -4 penalty to try something fun.

Then he started talking about laying on the ground flat with a greatsword and making reach attacks; in my opinion, if he wants to try, and takes the penalty for being prone and the penalty for making the attack he's welcome to it.
I also agree with being consistent, however; it's why I gave the benefit of the doubt to another player in the group when I couldn't remember if scrolls provoked op-attacks. I allowed the op-attack, and as I found later, he was indeed correct. So players knowing the rules is NOT a bad thing, it's just that arguments in a group need to be limited out of respect both for the DM, and the fun of the group as a whole. The whole argument about "DM is final arbiter" is not about some Dungeon-Master power trip; it's about harmony, moving the game forward, and keeping the pace at a fun level. Right or wrong,
somebody's gotta be where the buck stops.