Check, check and check again. When your weight is in the saddle the girth will become looser simply because you are sittiing in the saddle depressing it and making the girth loose.
Don't have it so tight the horse cannot breath or move, there is no point in making sure your sadle fits, you are absorbing the movement of the horse and following him if you do his girth up so tight he is sore.
The better the rider the slacker the girth can be. With novices on board you cannot take risks...make it firm. With more experienced riders, well, if they cannot tell if they are slipping out the side door........?
Your horse will tell you if something is making him unhappy, it's not the girth going up, it's usually the saddle starting to bear down that makes them flinch. A pinching saddle will make the horse flinch as the girth goes tighter, with some anyway, worth a check.