i usually sell when the value of bitcoins or altcoins  goes high and buy when there a price drop (vice versa) but still it totally depends on you if you need cash for emergency or something important events like birthdays, wedding, and any other occasions then youd probably sell and withdrew some of your coins but if you have a spare cash and doesnt need that much money, then its better if you could just leave it there on your wallets or on online exchanges.