I think that you should sell all your altcoins before the fork simply because you can expect a massive increase in price of Bitcoin after the fork and when prices of BTC goes up, there is a blood bath in altcoin market. You can expect huge amounts of losses if you are willing to invest in altcoins and the prices of altcoins will surely fall.
You can easily expect a price of 6000$ for Bitcoin after the fork so play smart, and dump your altcoins.