Im holding on bittres
By holding on an exchange you're unable to receive rewards, but I assume you're OK with that. If you're asking whether you'll lose access to your PIVX after zPIV implementation if you leave them on the exchange, the answer is 'it depends'. Since you're on Bittrex, and Bittrex is generally on top of these things, you are 99.9% fine. If you had them on e.g. Yobit, you'd probably be screwed since they take forever to update wallets (if they do it at all). That said, even if an exchange was slow to update its wallets, you would regain access as soon as they eventually got around to updating.
I guess the broader point is that by holding your coins on an exchange, you're putting your fate in someone else's hands. Bittrex is trustworthy so it's not a big deal, but the only way you can be 100% certain is to hold them in a wallet for which you control the private key.