The one major rule-of-thumb for me in making decisions as a GameMaster [always capitalize: it may be the only bennie...] has been:
If it's funny, it probably works.
The truth of the matter isn't in the "funny," but in the "Does it get the [appropriate] reaction?" This is true for any expression: writing, singing, making many levels of choices... if it's for your own catharsis or kicks, sharing it with others may not be the best use of your design. (I'm all for public masturbation, provided consensual indulgence on the part of the audience.)
There's a lot of GMing that has to be done from the proverbial "seat of the pants." When I make a ruling, irrelevant of the roll of the dice, I try to keep foremost in my head, "What will make the player jump?" (Or laugh, or cry, or...) If it's a negotiation roll, heck, I once ran a Shadowrun game where Mr. Johnson flubbed his roll terribly... he gave the runners an unlimited expense account.
If I recall correctly, they bought shares in the competing company, expecting the stock of the corp they were working for to suddenly plummet. They bought "supplies" of dubious value... and one runner bought the editing of a pornographic video to include himself and a known Dragon... did I say he bought editing? He bought himself an Enemy...
It was a one-shot so I didn't really concern myself too much with the details, but sometimes doing things a little over-the-top is good.
Many of the stories I hear about gaming are all similar: the amazing roll of the dice, the quip that has the opportunity to repeat itself, and the GM who lets it happen... 'cause it was funny.