Should you arch your back during exercise?

Answer 1

It depends on your goal.

Powerlifters use a full body movement to maximize the amount of muscle used in the bench press. Their body position uses a substantial arch in the lower back. Look at pictures of powerlifting meets to see the the range of the arch. A proper powerlifting position and alignment takes good instruction and practice.

Bodybuilders use the bench press to isolate the pectorals (chest) as well as the triceps. Many times a bodybuilder will lay "flat-backed" on the bench with feet on the bench (not the floor) as well. Many bodybuilders believe this helps them concentrate more on isolating the chest. Although most strength coaches would advise you to keep your feet planted on the ground.

Bodybuilding is typically safer to learn on your own with all sorts of material on the internet.

Answer 2

not on bench press