TBH it's up to you!
What are your current poker related goals? If it is to move up limits, then I say use the cash to pad your roll - essentially it is a return of money that you've had removed from your winnings anyway so I never mind using it in this way.
However if you make periodic withdrawls from your roll, then it seems to me cashing in your reward points is a nice method of having a semi regular tangible return.
I guess you have to decide if you want to reinvest your capital in your game, or utilise the return on investment as income!
Hopefully it goes without saying how you are running has absolutely nothing to do with how you should utilise the cash. You have to have some idea of how you want to use this money before you earn it (or cash it in I guess) that is unrelated to your recent results.
Personally, I had intended to reinvest all rewards/rakeback et al back into my BR to try and get it to a comfortable level for attacking higher limits.. but after a recent rethink I've decided to pad my roll by getting better and withdraw these funds every week or two - possibly reinvesting some if I think I should be moving up limits in order to get there faster.
Dunno if that helps at all - at least rewards money is not as subject to variance and can therefore be relied upon a little more!