don't think much about this because if you buy altcoins with ETH then if you want BTC then sell altcoins with BTC so no need think much about, its depended by yours
If you are having some bitcoins in your wallet and that you are planning to sell those bitcoins to have some altcoins, then I must suggest you to reconsider your decision as this is nit a smart choice and doing so can then put you in trouble. You have to look for other ways which can then help you in buying some altcoins but this way, you are going to lose most of your bitcoins.