For me it's a matter of where the waistline sits. Wearing a traditional 2" rise kilt, especially as a smallish guy, can make the line between upper and lower garment look a little high. This is a good reason to wear a belt or waistcoat, but untucking a tshirt or maybe a polo shirt can drop the waistline down for a better overall if more casual profile. Untucking a button down shirt with a collar can present the opposite problem. These days such shirts tend to have short 'tails' intended to help keep them tucked and this shirt will hang too far down and look sloppy in the same way that a sport coat with a kilt looks too long and sloppy. These issues become less relevant with a more 'modern' profile of contemporary kilts. Of course the level of formality in dress must be considered and I would never untuck if I need to be perfectly presentable.