Three Issues to Keep in Mind when Changing the Rules.

[Haltherrion]

I'm sure it's a DMing style thing, but at least in my case, I try to keep players in the loop with respect to rules changes and ad hoc decisions. I give them my ruling and rationale, they can protest or offer an alternative ruling, I make a final decision, and we move on-- with the promise that I'll review/correct the decision & make it official between sessions, especially if I know it'll pop up again. In session, the whole process takes 2 or 3 minutes, and is pretty painless. (Perhaps that's what you meant by "gaming the DM" in the OP?)

I agree. I think a little give and take with the players is the only way to go. For one thing, they may find an important aspect of the rule you over looked.

But more fundamentally, we the gaming group rarely have a way to "prove" the validity of a rule. We don't do mythbuster type stuff and run out and test every theory. We basically create rules that feel appropriate for the situation at hand based on what we know about the situation and the game-mechanics. There's no point in having a rule that feels right to the ref and wrong to the players. At the least it causes potential for in-game stalls (as it gets argued every time it comes up) and in the worst case it can cause irritation sufficient for player drop out.

You make a ruling that seems sensible. You test it with the players. Based on play-test or maybe even someone joining the group with a new perspective, you adapt the rule. For example, you might make a ruling on catapult reload times that seems fine for everyone in the group. George the military history buff joins the group and has data based on experiments with real catapults or reading of mil history that suggests you were way too optimistic, you change the rule. No harm done. You move on.

There isn't a platonic ideal rule you are working to capture. It isn't a fantasy world simulation. It's a game with a thousand compromises already existing in the rules to make it playable and enjoyable.