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