As a traditional kiltwearer of 20 years experience, I'd say as a Scotsman one should try and maintain tradition and go regimental - it's expected and can be achieved without embarrasment given experience. However there are occasions when the wearing of the kilt for prolonged periods of time can get up an allergic reaction and whereas creams can be applied (see previous postings and censored thread) it is useful to wear something to allow the rash to die down. Also in the company of children and family and indeed work colleagues it is often desireable to feel confident that there won't be unnecessary exposures. A recent survey on www.sporranclan.org discloses that kilt wearers 87% go regimental, 2 % wear a thong, 6% a jock strap, 2% black panties and 3% anything that comes to hand.
So what's my preference: Boxers are definately no go. They are more likely to be seen. Some wearers have disclosed the most tasteless articles I have ever seen and deserve to be shot at dawn - if they are to be worn at all best to be lycra and close fitting to avoid detection.
I'd approve black panties (not for me) but they are tasteful and worn by dancers in competition (excluding military).
A black or mesh thong is good 'cos if the kilt lifts at the back no one thinks you are wearing anything and thus to all intents and purposes you are regimental (and rarely does the front get lifted due to weight of the Sporran). I find jock straps confortable, but that is a personal preference (Good if one is doing a lot of photo shoots and has to bend down frequently).
Have a look at www.internationaljock.com there is much to choose from there that might aid a discrete cover up and support.