If you disable something critical, yes, you can hurt performance. You can even make the system unusable.
A lot of those unwanted apps running in the background can be killed without anything bad happening when you're not using the product that they support. Some, like mail and anything else that checks something on a regular basis can be safely shutdown too, as long as you do it while they're idle. But you won't have your mail checked automatically until you restart the service. Some you don't want to touch, though. So know what you're shutting down first.
Unless you're having problems and are troubleshooting to find the offending app or service, you might as well leave them be. Look at how much RAM you have and how little RAM is being used. As long as you have plenty of free resources, killing those processes aren't going to gain you any performance.
