I've seen boats that when you lift one bow the other won't move until the one your lifting is 5"s in the air. If you don't start with the beams level to each other then you risk gluing the boat in a racked position.

On a modern well built boat some of that might not be needed but if someone has me do it then I'm going to go for best result possible. Loosening the striker lets the beam flatten out and makes it easier to get a level on it. Also not a bad time to take the striker apart and service it.

On a older boat that's never had the beams set it's very possible to find it out of square. Not much but I have been able to move them into position. Add to that 5" bolts in a 1/2" aluminum plate with worn threads. They move around.

One more thing about using gel coat. It shrinks quite a bit so I don't see it being the best stuff to use.