While it's true that USB ports are supposed to be "hot swappable", I'd strongly recommend against trying to plug something into one on the motherboard unless it faces out through a hole in the case. The reason is that if it's somewhere in the middle of the mobo, and you happen to touch anything else, you could cause your system to crash and possibly damage a component. Even with the power off, you have to be careful about because of static charges. When there're so many millions of circuits on a chip, the tiniest of voltages can cause damage.