You get a +1 to Parry using a Rapier. If you hold a Rapier in your Off Hand do you still get the +1 to Parry?
Yes, it does
The only penalty from using a rapier in your off hand is that you can't use that hand for something else. The +1 parry still applies. Source: Official Rules Forum
Also, I second @aramis as another former SCA fencer. Makes total sense.
You probably should. It's not significantly harder in Real Life to parry with the off hand rapier than it is with the good hand.
(I was for 10 years a SCA Rapier fencer. Hope to be again.)