One of the toughest decisions a player has to make when serving under pressure - or at any other time - is whether to go "macho" or "tricky." Here's a Tip of the Week on the topic, Trick Serves and Third-Ball Serves, with third-ball serves the same as what I'm calling macho serves here.
Going macho means you serve mostly to set up a third-ball attack, knowing that you will have to follow it up with a strong attack. Most often these serves will give the server a return he server can attack, but the receivers generally don't miss these serves outright, and the server does have to make a good shot or sequence of shots to win the point.
Going tricky means pulling out a serve where you are trying to win the point outright with the serve, either by the receiver missing the serve or giving an easy pop-up. The weakness of these serves is that if the receiver reads them well, they are often easier to attack then third-ball serves. They tend to be all or nothing serves, where either the receiver misses, or the receiver attacks the serve. (Of course, if the opponent plays passively, then things are different - challenge them over and over with trick serves, since there's little danger of them attacking them.)
So which should you choose? First off, you should have complete confidence that you can win by going "macho," knowing that you can follow up your serve. At the world-class level, theses serves dominate. At the same time, you want to play the percentages - that's all tactics is, playing the percentages, and so that's all you should be thinking about here. You can't put an exact percentage on it, but you should be asking yourself, "Do I have a serve he keeps missing?"
Sometimes you might even have a serve you haven't used yet, and pull it out at the end - but I don't recommend that. If it's such a good serve that it should win you a point, use it earlier, both to win the point and to verify it'll work. If it does, then hold back on it for a time until the opponent isn't ready, then pull it out again. Why pull out a tricky serve at deuce when you could pull it out early in the game and perhaps again in the middle (and perhaps not go to deuce), and again late in the game if needed?
Some players prefer to go to simple "macho" third-ball serves under pressure because they are more likely to give predictable returns, which simplifies things. And most of your serves should be of this type, with the trick serves thrown in for a few free points.
So it's all about the percentages. With experience, you'll get a feel for which type of serve to pull in those pressure-packed moments.