It’s the matter of preference which length of hair you want to have under arm pits. Either you have a wild bush or clean shaved pit; there are some rules you should stick to.
Hygiene is really important, not only it’s easier to wash it off when you are clean shaven, but the deodorant can be put directly onto the skin and not on underarm hairs. Furthermore, you’ll sweat intensively with a lot of under arm hair and grow more odor-causing bacteria.
Take a note that when you see hair peeking out of your short sleeves, it’s time to shorten it.
The best way to permanently remove the hairs from arm pits is by hair electrolysis
Unless you are a professional swimmer, runner or cyclist do not shave whole arms. Some bodybuilders shave their arms to achieve better results at competitions.
Trimming or going bare on your shoulders and biceps is totally fine, especially if you like to show off your muscles.