I stick a sock in my underwear when I go to the gym (not all the time though). That's just so people won't notice anything weird when Im laying down.
Sometimes also when I'm wearing khakis or dress pants,they tend to outline the crotch area.
Otherwise, I dont really have a reason to.
Although, I get aroused when I actually have something in my pants that feels and resembles a penis. Even thinking about it is really arousing for me. I wear it at home when I'm alone just for the imagination.
Since ill be living on campus in two years, I plan on getting a lifelike prosthetic for about a grand,that has an STP device built in. That way, I wont have anything to worry about and can be seen as a typical male, clothes on and clothes off.