there have been debates on what works better. The consensus is that it is entirely player dependent. There is no "right way" to jc grab, it's just what you feel comfortable with.
A clearer definition of the term is canceling the jump animation with a grab. So you start the jump animation, which involves your character bending down preparing to jump, and then extending his legs to jump and eventually getting in the air. This happnes all fairly fast, so it's hard to see it in these parts, but just be aware that that's how it works. Now the object of jc grabbing is to hit grab while you're in the windup of the jump animation, while the character is still on the ground, but preparing to jump. The main utility of jcing a grab is to run and then jc grab so that you don't have to deal with the ****ty lag time of your character's missed running grab animation. There are also little advantages like different grab boxes and speeds, but mainly it's minimizing lag.
Hope this helps.