Players often hold back on their trickiest serve (often long ones) until it's close. And that's sometimes a good idea. But there are three problems with that.
First, you won't know until you use it if the serve will be effective against this player. Second, if you use it early in each game, then that game might never get close. As long as you space it out, a tricky serve stays effective, so you don't need to hold back on it too much. If it's effective the first time early in the game, then perhaps come back to it in the middle of the game, and perhaps at the end again. And third, once the opponent knows you have this tricky serve (usually long ones), he has to guard against it - making your other serves (often short ones) more effective!
If a student of mine has a tricky serve - often a long serve - I regularly tell them to use it the first point of the match. If it works, then it's a judgement call how often to use it. The more you use this serving tactic of using your trickiest serves early, the more experienced you'll get at judging when to use them.
One cautionary note - if you want to reach the advanced levels, over-relying on tricky serves - especially long ones - can really hurt your development. Focus on shorter serves that won't give you as many outright serve winners but will set up your attack. Then mix in tricky serves for free points and so the opponent has to watch for them.