The ideal shirt length for a guy, whether it's a button down, a polo, or a t-shirt, is halfway down the zipper, or from the back, covering your rear fully. This is to ensure that when you sit down, you do not untuck your shirt accidentally.
Because of its length, try not to untuck your shirt. Not even after work when you are having your beer. You will look quite silly with that long tail flapping around your ass. Trust me on this, I have been there before.
Terrible looking dress shirt that is untucked
Shirts meant to be untucked have a square cut, whereas office dress shirts have a rounded cut at the hem, to make it more suitable to be tucked in.
Note the square cutting of the shirt hem. Look out for this when you intend to wear the shirt untucked
If you wear something over the shirt - a sweater, a vest, a jacket, a blazer – tuck your shirt in, unless it is a properly fitted t-shirt.
If you are wearing a short-sleeve shirt, the sleeve should rest in the middle of your bicep, halfway between your elbow and shoulder.
If you are wearing a long sleeve shirt, the sleeve should end in the little gap on the side of your wrist (or, alternatively, about ¼ inch below the wrist bone).
If you are wearing a button down, the buttons should not stretch the shirt when buttoned. The collar should fit snugly; but not tightly; whether buttoned or not.