They can develop a habit of attaching themselves to the sides of discus and suck their slime coats off, causing at best high levels of stress, also leaving the site open to infection. Once they've done it once, they will keep on doing it.
However, it's something that a lot of people say is very rare, to the point where you don't need to worry about it...
Bet that doesn't help much!!! You could try posting in New World Cichlids though.
Personally, as you've already got them in place, assuming you couldn't easily rehome them, I'd just keep a close eye on things and remove plecs if there is any sign of a problem.
