algae blooms that come from tap water in a tank where you are trying to closely observe a fish can be a real problem. also if your QT is your HT (meaning if a fish gets sick n QT you medicate in the same tank) then water quality issues can easily cause your (now) sick fish extra issues.
Having said all that, I wouldn't be TOO freaked out about doing it, but with the size of a QT, the time/money spent on RO/DI sure doesn't seem to outweigh the benefits of using it.