So, it's important to ground it because that's going to remove ground hum if you have it. But beyond that, in order for it to be grounded properly, you should ground it to your head unit or your mixer. If your mixer is grounded, then that's fine, that would work. Failing everything else, yeah, the nearby computer would probably work okay. But ideally, you want it grounded to the piece of equipment you've got it connected to. Incidentally, you'll know immediately if connecting it to the computer was a bad idea because if there's a difference in the ground voltage between your computer and your mixer, you will hear an immediate introduction of a humming noise or buzzing noise into the audio.

Either way, it's not really a safety issue, or shouldn't be, if you don't connect it to anything.