Is it safe to hold altcoins for long-term in exchangers like Poloniex or Bittrex? Or should I store the altcoins in their respective wallet?
Oh, i'll recommend you to use hardware or software wallets. There are some reasons for it:
1. Exchange can be hacked.
2. Exchange is a quite popular subject for DDos attacks.
3. No control over your money.
If you really care about the security, use hardware wallets (like Ledger or Trezor). They are the most secure and perfect as a long-term storage.