If you have a lot of USB devices running through 1 port and spliting to multiple Macs, I guess a switch is necessary, but otherwise, I think a hub is enough.
Looking at the USB plug, there are only 4 pins, you figure they are power, ground, in, and out. Seems to me like there's no handshaking/error checking so shouldn't be a problem to share devices...of course all of that is theoretical, I'll find out when I setup a scanner for 2 iMacs later ^_^