Suppose you have a tricky serve that gives your opponent all sorts of problems. But suppose it's also one of those serves that he can get used to, and only works by either surprise or by the opponent's not being used to it. Should you hold back on this serve for key points in the match?
To start with, you won't really know if the serve is effective unless you actually try it out against an opponent. Sometimes you may know from previous matches. Either way, you should use it early on to establish whether the serve is effective. If the serve is not effective, then you know not to use it later on. If it is effective, you get to use it early in a game, and then come back to it several more times, including near the end if necessary. If you do need to come back to it again near the end of a close game, imagine where you'd be if you hadn't used the serve early on, perhaps more than once?
The idea behind holding back on a serve is that 1) if you use it too often, the opponent will get used to it, and so 2) hold back on it until you reach a key point. The thinking is that if you use it early in a game, and that games ends up not being close, then you've "wasted" that serve.
There is logic behind the above, and yet many players way overdo this, holding back on their best serves - i.e. some of their best weapons - except when it's close. This is like a looper not looping until it's close, or a hitter waiting until it's close before hitting.
If you have a serve that really bothers an opponent, use it regularly in rotation with your other serves, and put yourself in a position where you use the serve at the end of games to win that game, not just when you are down (and where you might "waste" the serve anyway if your comeback falls short). If you are in a competitive match, and can win one "free" game on the strength of one serve, use it! And don't kid yourself into thinking that your opponent will magically become strong against this serve as long as you use it somewhat sparingly and with some variations. His returns may become more effective, but that's true of any shot you may throw at him.
If you do have one serve that really gives the opponent trouble, try to use variations of it to keep him guessing throughout. The more you vary it, the harder it will be for the opponent to get used to it, and so you can use the serve more often.
It's better to use your winning shots (and serves) and win then to hold back on them and hope to come back and win with them at the end.