Undies under bike shorts give some folks I've ridden behind some really unflattering panty-lines.

I've worn UnderArmor compression shorts under my bike shorts. It was pretty comfy, kept stuff fairly clean, and no pantylines.

But I generally prefer to go commando in my Terry commuter skort and Ibex capris. I have two very old pairs of plain bike shorts, and they have become so large on me that I don't wear them anymore. Save 'em for laundry emergencies now.

I like my Terry skort so much that I'll probably buy another one instead of a pair of shorts. (and how long can a girl go with just 2 things to bike in? gotta shop!)