So. You've decided to make a shirt. You want to wear it tucked-in for work. You also want to wear it untucked for the weekend. You get two shirts.
A shirt meant to be tucked-in is cut quite differently from a shirt meant to be left untucked.
SHIRT (A): untucked SHIRT (B): tucked-in
Below are the two major differences.
1. SHIRT TAIL LENGTH
For a shirt to be tucked in properly and remain in, the body has to be cut substantially longer. If not, there is a high possibility of it becoming untucked when you raise your arms. After you've helped that lovely lady reach for something on the top shelf, she does not need to see your shirt getting untucked in the process and exposing your Calvin Kleins.
Shirt (B) will just look ridiculously sloppy when left out due to its longer tail-length. In our view, the front of an untucked shirt should never fall past the halfway-point of your trouser fly.
2. BODY SHAPE
Shirt (B) is cut with an hourglass body shape to fit the torso more closely. However, when left untucked, the ends of the tail will appear to flare outwards, much like a ladies' shirt.
An untucked shirt should be cut rather straight in the body, refer to Shirt (A), so that the fabric will fall and drape properly. A gentle suppression in the waist is fine, but if the suppression is too obvious, it will end up like Shirt (B). And speaking of drape, heavier cloths will almost always drape better than superfine ones, remember this. Think oxfords rather than smooth poplins.
Since we have to maintain a rather straight cut, almost the entire body must be brought in to cater for a slim-fit. However, this will result in a tighter chest, which might in turn result in pulling across the buttons and catching/creasing near the armholes - sorta like what Dr. Bruce Banner experiences right before turning green.
What happens when you tuck in Shirt (A)? All the excess fabric will balloon out from the waist. And don't raise your arms.
All the complications are somewhat inter-related. Therefore, unless you are willing to compromise on all of the aforementioned problems, make a decision - either you're in, or you're out.