You don't have to pay for exit rows in Premium Economy, only Economy (on certain flights, not including the MCO routes for example).
However, there seems to be some ongoing confusion - especially at remote airports - where some check in staff are under the impression that you do have to pay for exit rows in Premium. I have double (and triple) checked this with Virgin HQ, so if you are told you need to pay the exit row fee in PE, politely ask the check in agent to confirm that with their station manager, since it's a common mistake. (BTW, when I say 'politely', that's important, since you live or die by the check in agent, and if you tick them off they're not going to go out of their way to release the exit row anyway. I'm sure they have to encounter plenty of not-so-nice passengers who think they know their jobs better than them, so not a good idea to be confrontational

). Also, policy at Virgin may change in the future, so if the agent does check with the station manager, and they still say the PE exit row is chargeable, then assume there's been a policy change.
Getting those exit rows normally means getting to the airport early, and you have to be physically able to do the job of opening the doors should the worst happen. I remember travelling with a couple of friends once, who at check in were being assigned the exit rows; but when one of them said, 'Ah, excellent - I'll be able to stretch out a bit, which will be good because of my bad back'. All of a sudden he lost the exit row, and wished he hadn't opened his gob

Pete