That is the worst you can do to store it in an online wallet. If you are meaning online web wallet you are making a big mistake to leave more than 100 btc to some other parties as you need the website of these wallets to operate your bitcoins and these websites are not always 100% online. If you mean Electrum or Bitcoin core then it may be Ok but for the best always use a hardware wallet for such case.
why ?
Because if you do not own your own private keys then you are not really in control of your funds. Online wallets and exchanges let someone else manage your keys for you, if they lose them or steal them then you're out of luck.