If you have a tight or semi-tight grip, can you grip the cue exactly the same tightness every time? It's really hard to be exactly *THIS* tight every time.
On the other hand, if you're relaxed, and aren't gripping the cue, but rather letting it lay in your hand, *THIS* is repeatable and very consistent.
Another 'upside' to having the relaxed grip, is you immediately know when you're not and the reason you either missed the shot, or the CB didn't do what you were expecting it to do.
The last upside from the relaxed grip is what your cue will be able to do. When you grip the cue, you're robbing it of what it's designed to do. Transfer energy to the CB. With a tight grip, some of that energy will transfer to your grip and arm, and not to the CB.
As always.. this is free advice and you're welcome to do with it what you will. Good luck to you with what ever you decide.