I aim for being permissive, with two caveats:
1) Not setting precedents - I am cool with allowing a thing once. But when you ask me to allow something in the middle of running a session, maybe it shouldn't become routine. The wackier the attempted action, the more it will be dependent on the situation you were in at the time, and the less you should expect it to work again.
2) Not being exploitive - a new and different approach to something should be about as effective as things available in the rules, overall. Don't aim for permission to hit an "I win" button on one action. If you want to stack up some creative stuff to construct an "I win", expect a skill challenge or two.
I make liberal use of advantage ("Oh, you did new clever thing, you get advantage on this roll to do X."), and skill-challenge-esque structures to enable creative solutions.
Edit to add: There are games that are built out of permissiveness, because they give solid frameworks for it - Fate being a prime example.