In terms of storing altcoins, it's okay to leave them on Bittrex (can't vouch for all exchanges) until their individual values exceed some predetermined threshold. That threshold will depend upon the level of convenience desired and your level of competency.

For me, I would feel comfortable leaving any altcoin wallet worth a few hundred dollars on Bittrrex and sleep really good at night. If you get heavy into a particular coin, then you might consider using a local wallet and following best wallet practices.

No need to have dozens of altcoin wallets installed on your local machine. Opinions will vary.