I never have, and I've never even thought about it. On a recent holiday I noticed my dad put a rolled up sock down his Speedos when he was in the hotel apartment getting ready to go down to the pool. He didn't seem aware that I could see him.
Later in the day - and a few beers later - I made a comment to him about it when we were separate from the rest of the group and lounging by the pool bar. Said to him upfront that it looked like he'd stuffed a sock down there, and he just laughed it off (a bit embarrassed I think) and said yeah. I asked why and he just shrugged his shoulders and made a comment about maybe impressing any women that happened to look his way. "Just for show" I think is what he said.
Out of curiosity, I shoved a sock down my Speedos the next day just to see what it looked like. It looked a bit ridiculous so I obviously took it out straight away.