I would only store as much in an online wallet as you would carry cash in your wallet...
Just like cash - online wallets are very practical - but not 100% safe.
Remember online wallets are NOT REGULATED. They are not like a bank - they do not have auditors or compliance officers etc... In fact they can just be some undergraduate sat in his bedroom.
If you are going to keep more than this - say what you earn in a week/month then do some due-diligence on the company - at least check they are more than a one-man-band and there is some hope they get someone to PEN test them (i.e. an external expert to try to hack them).