So If you let them [i]try[/i]...
So, are you guys actually letting them try, or are you making the mods so prohibitive that you may as well be saying no. Essentially, are you taking on board their offer? Or are the consequences pre-determined "unless they roll a twenty."
For instance, what if a player - in a really tough situation - asks if they can try a variant of a spell that probably shouldn't be allowed under a strict interpretation of the rules? Normally they wouldn't try it, but this is an emergency.
Is your answer...
Tough, it doesn't work? (Maybe you are worried about them doing all the time in the future)
Yep, but you need to roll a natural twenty? (Which frankly, is pretty much the same thing as the above)
Okay, you cast the spell, and this happens!? (You allow the attempt, they roll, and it works, but not everything goes to plan.)
Sure, and you can have a bonus to your die roll for the nifty idea? (You don't give it a second thought. After all - as the GM, there is always tougher stuff you can throw at the players. And you don't want them to feel impotent. )
Because sometimes yes means no and even though you think you are saying yes, sometimes you are really saying no... you know?
To put it another way. Are you encouraging your players to act creatively?