I like andyfg's son's "out" look. I don't think it looks tacky or unkempt, I think it is casual. Although when I dress casual I tend to tuck the shirt behind the sporran a bit.

My kilts are all tranditional height, which looks great for formal occasions, but in the Midsweat, I like to have a bit more breeze through my shirt. If I wear my kilts at the proper height, and tuck a T-shirt in, I think it looks goofy. Imagine pants up to the armpits look. My kilts are about 3 inches above Richland's on my waist.