I wouldn't leave crypto on an exchange unless you were trading it. Just transfer it to a wallet as soon as you buy it. IOTA is a looooooong hold. Their wallet is solid and there's also a paper wallet option. - Can't confirm the bittrex addition, but this was one of the rumors:
Who knows if and when they'll add it. I'd imagine the fact that it's not a block-chain is part of the hold up.
I am always trading. Buy and hold is not my approach to investing. So would rather not move it in and out of a wallet each day I want to trade.
Once I hit a goal amount in the trading account for a coin then 50% of profits get moved to a wallet each day. This is then my holdings, but in crypto this hold could mean days, weeks, or longer...just never know. If I feel there is a really good opportunity in another coin.