I don't play with optical triggering that much.. but I assumed that the red front part of the flash needed to be facing your subject... at least that is what I did when I use it. and with the fully articulating head of the 580 or 600, it seems like the flash doesn't HAVE to be in front of the camera. I could be wrong on all fronts though.
Optical trigger uses visible light, that is the actual flash head.
In case of 580 or 600, the raw strength of the light might be able to bounce and activate the slave, if needed. However, the maneuverability of the head is useful since a direct line of sight is ideal and in case of a puny master like the 90EX, it is necessary. I can't imagine how restrictive that will be in terms of placing the slaves.
Thanks, PBD, you saved me 60 bucks!