I have never had a problem with an Otocinclus (Oto for short) and include them in all my discus tanks. Some other types do get a taste for sucking on discus slime coats. Some people have also had the same experience with otos, but I haven't found this the case. If you do see the algae eater sucking on the discus, you must remove them. Once they get a taste for it, they will harras the poor discus no end.

Make sure it is a true oto you are buying, as a LFS will often mis label and sell a common sucking catfish as an oto.