It's indeed personal preference -- but I'd say whichever you choose, look in the mirror before leaving the house. I really wish more people did that (and not just kilt wearers). I'm not a fashionista but seriously, you'd think some people never heard of a mirror before.

Having said that, I always tuck my shirt into my kilt, mainly because it defines my body more. I'm not a sculpted bodybuilder or anything, and I try not to wear muscle shirts since I don't have six-pack abs (yet) but I do like wearing my clothes in a way that gives my body a bit more definition.