Nah, it's just a natural movement really not a conscious decision to "jump". Same as when you hit a big forehand for example, you load up your power then explode into the shot and you will elevate slightly. Try jumping and swinging and you'll screw the shot up royally. The only shot in tennis you should consider jumping for is when you're reaching for a high lob or executing a slam dunk!
Yeah it isn't really "jumping." Like Bernard_Black said, it is natural to come off the ground when you put every ounce of your weight into the shot. When you become a good, solid server, you SHOULD be coming off the ground because that means you are using your legs or back to throw your weight into the serve.
Brian Battistone ! Crazy ! I jump but I don,t just really intend to jump I just reach for the ball. This guy at a club I play at jumps and it,s stupid as it adds nothing. he doesent even bring the racket behind his head when he serves ! He can also jump about 2 feet in the air , does this at net . He,s irritating but easy to beat as he,s new to the game .