I've always used the principle of "Consider Yes" at the very core of my DMing. I run the game for the enjoyment of my players and myself. If one of my players has an idea that they want to try that will make the game more fun for them, I will be inclined to say yes unless I have a very good reason to say no (for either rules balance or setting flavor.) This is a game, and I can think of no other reason for playing the game than to have fun, so anything that makes the game more fun is a good thing.
And remember people: the wealth by level guidelines are just that; GUIDELINES. They aren't a hard and fast rule, and your game isn't going to come crashing down if the characters have a bit more or less than the guideline says. If they have more, then use opponents with a CR of a level or 3 higher against them.
Don't become so obsessed with the letter of the rules that you lose sight of the reason for playing the game: it isn't "to follow the rules", it's "to have fun." The rules are there to facilitate fun, and when they interfere with having fun they should be altered or discarded.
For too long, my primary DMs default answer to anything was "No" and it really made me question the point of making things less enjoyable for people in a game we all play for fun. He has since become more open to options and has consequently become a much better DM for it. (I'm not talking about things like letting players have dragon PCs, but things like allowing the Warlock class.)
Remember, this is a game, and the point of games is to have fun. Having "No" as a default answer for everything isn't very fun. Consider options and how they could fit into the game, and if they don't fit feel free to say no, but if you can make something work and it makes the game more fun for one of the players, well, that's the main purpose of being a DM, isn't it?
