It's not exactly safe to store your cryptos in any exchange and just leave them there. Your best bet is getting a hardware wallet
If you are a short term or daily trader than it is safe to an extent as Binance in today's time is one of the reputed exchange among all the existing one's. If you have huge amount of btc or a long term investor than the offline mode is only the best rest will be a risk bet to store it in online wallet.