The only shot in squash where you must be touching the floor is the serve. For that you need to have at least one foot inside the service box, but not touching the line. So, unlike tennis, where jumping is part of a good swing technique because it gets you higher, squash you have to stay on the floor.
Now, I have talked about diving in squash before, and that is allowed, so clearly you don't need to be touching the floor when you make contact with the ball, but diving and jumping might seem different. In the past, I has been asked this question, so it seems that it is something newer players wonder about.
Is It Worth It?
The only time you see advanced players jumping is when they are around the middle of the court and their opponent hits a lob that either is terrible and can be reached by jumping or is a great lob and it seems that if you let it bounce it would be a winner.
Some players would use this opportunity to jump, but in reality jumping is very, very rarely needed.
Video Analysis
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In the first case, you can probably play any shot you want, whereas in the second case most players try to hit a spectacular crosscourt nick winner. I’ve seen it done and it is spectacular – when it hits the nick, but if it doesn’t, it means you take longer to reach their return because you have to land from your jump and then move towards the ball.
Jumping does make hitting a nick a little easier though as the point of contact is higher, which means the angle is more acute making the margin of error better – at least in my experience.
Should You Jump?
No, I don’t recommend it. Neither do I recommend diving. For most club players jumping or diving, really doesn’t improve their ability to hit good shots or reach balls they normally couldn’t.
Spend more time on developing strong legs and core, stable footwork and adaptability. Like a few other things the pros do, it’s cool to watch, but much harder to actually do. As with all great performers, they make those things look easy – but they really aren’t.