"while you're accessing your data"
That's really the critical part, isn't it? If you're using this for backup you should never need to decrypt it. The only time you need it is if you have a local failure. Then you have to make a choice: give up the data or take a chance that they are at the server siphoning off your data as you request it.
For 99.999999999% of data, I'm going to say that the US government doesn't give a fuck and the chance that they're monitoring your account when your local copy fails and you are getting your data is going to be pretty darned near zero *unless* you happen to be the target of an investigation. If you are, I would suggest that you pay the extra money for something like SpiderOak, where all the encrypt/decrypt is done locally. Though, to be honest, if you're going to be watched by the Feds, a USB drive and a good fire safe is probably a better solution for backing up your "sensitive" data.