I only wear t-shirts with my utilikilt and I never tuck them in with it. Reason being that it sits at the same spot as my shorts and the shirts just pull out around the edges as I move and I hate the way that feels/looks. I do have a white linen button up that's flat across the bottom with no shirt tail and thought it looked stupid not tucked in but it's too short to tuck in so I don't wear it with the utilikilt. With a tartan kilt if I've skipped the kilt belt and sporran I also won't tuck a t-shirt in as the kilt will show too much unbroken up surface, as a regular kilt does sit higher if I'm wearing a belt which is more often then not I will tuck a t-shirt in as the kilt sits high enough that the shirt doesn't pull out.