At close range, Ken has a basic high/low mix-up to land super. He can do short short super which hits low, or far stand strong super which hits high. Against most characters, if they try to parry high, they are vulnerable to low short; and if they parry low, they are vulnerable to far stand strong. This mix-up can be done at wake up, after a low short or stand strong tick, or just walking up to you.
However, small characters don't have to worry about this. If they guess-parry low, Ken's far stand strong whiffs right over them. Therefore, it is beneficial to parry down in this situation. If Ken does low short, you parry his move. If he does far stand strong, it whiffs and you are at an advantage during his recovery.
For example, if Ken does low short as a tick (not short x3, but just 1 short), immediately parry down afterwards. Down parry will cover low short, low strong and low forward, and far stand strong whiffs. He will have to physically walk up and do his chain combo, or do overhead to hit you. In either of those cases, you have time to go from low parry back to blocking to cover yourself.
Parrying low against Ken after ticks at close range is really useful. You are covered for everything except throws. Chun Li, Yun, Yang and Ibuki should do this often against Ken.
There are other characters where this can come into play, but not as useful as against Ken. Against Chun Li for instance, her far/towards roundhouse goes over small characters. However, her back fierce will still hit, so it is not completely safe to do.