What kind of box are you using?
Plextor originally sold the Premium and the Premium II in metal fanless enclosures (while the Plextor DVD drives were originally sold in plastic enclosures with fans), so it is probably fine. Then again, adding a fan won't hurt anything and it might help, so why not?
I always go back and forth on the fan issue. I definitely think keeping the drive cool increases its life span the burn quality...but then again, in most optical drive USB enclosures, there is no real air circulation so the fan doesn't really do anything anyway. I have enclosures with fans and without fans and the drive seems to get rather warm either way.... Now the Plextor 716UF enclosure had a nice fan design, where air was drawn in from the bottom of the enclosure and sucked out the back...but finding well designed enclosures are hard!