weeeeellll - just to throw my two kroner into the fray: I commuted for years as a poor student in "regular" sportswear (anything stretchy I had around), then in my one pair of bike shorts with underwear underneath. I never had problems, but every now and then I'd be riding more than usual and feel a bit chafed or sore but just blame it on my saddle.
Fast forward to today, I'm on my bike for longer and more frequently, I ride to train as well as commute, I have a longer commute and take detours, and ride hard enough to need a shower after most rides. Not least - I can afford and have bought multiple bike shorts, and after wearing a good pair without underwear have never gone back. Having had a really horrible saddle sore once made me fully appreciate all the comfort I could get.
I'd say it depends on your riding style and/or the quality of your saddle and/or the sensitivity of your skin and/or how much you sweat... You certainly can wear underwear underneath, I did so for a long time and it does save your shorts from quite as much washing. But I agree with the basic premise which is that bike shorts are made to be worn without underwear and work best that way, so if a beginner asked, that is what I would answer too and I'd recommend they try without. But hey, whatever works.
Oh well. This thread is 4 yrs old as pointed out, and I'm guessing anyone reading this has made up their own mind long since
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett