Most Helpful Guy
Most men's shirts are designed to be tucked, actually. If the shirt's length covers your pockets, it should be tucked. A shorter length, untucked. Untucked is very casual.
I dress in layers--looks sexy ;) --so I usually tuck. When layering, the outermost layer should be longest, inner layers shorter/tucked.
Plus, when you are tucked, you can sport a fashionable belt that can really make an outfit.