When designing new adventures, you should probably be pretty strict. When launching a campaign, you can be somewhat looser--but not too loose. Here's why:
Let's say you have a character who wants to be a performer--someone who's good at many types of performance. (Perhaps he's basing his character on someone like Harry Connick, Jr.--a person who sings, plays a couple of different types of instruments, and even acts. He isn't great at everything, but he's great at some things and at least competent at some others.) With the existing list, the player has eight skills to spread his skill points around. If you increase that without increasing the number of skill points characters get, you restrict the player to a smaller slice of the skill pie, as it were. The more skills you add to the game, the narrower the focus of individual characters. That may be OK in your game (especially if you only increase the list by 1 or 2 skills), but be aware that it does narrow your characters.
That's why you shouldn't be too loose with this when launching your campaign. Here's why you shouldn't introduce new skills into adventures:
Players choose their skills from the list in the book. There's virtually no chance that you (as GM) will think up a new skill, and the players will happen to have also thought up (and taken) exactly the same skill when they were making their characters. If you encourage the players to think up new skills, chances are they'll spend valuable skill points on skills that won't match up with what you call for in your adventures. Likewise, if you introduce new skills in your adventures, there's no chance that players will have ranks in those skills. If the skill matters to the adventure, the heroes will be unprepared. If it doesn't, why bother to define the new skill?
I hope that clarifies the rationale a bit. As always, you're welcome to change the game as you see fit!
