As an astrologer, this can go either way.
It depends on other astrological influences - both natal and daily.
Those born with Merc Rx probably already know they have a tendency to say the wrong things, or not make themselves clear - the best thing to do is really make an effort to stop and think before 'speaking'.
Depending on what sign your Mercury is in, it doesn't have to be retrograding in order to feel these same effects.