That doesn't necessarily prove the addresses actually belong to you. I guess if your website says "Send money to: " and the "Our money is at:" are the same addresses it's probably safe to say they are yours. Unless... you add a fake address with lot's of coins just so you appear to have more than you do. But by doing that, you'd be losing some payments from your users to that other party.
That's an intersting point, so as new addresses are doled out to users to deposit, they are also published. But I also see that yes, you could have some address (that say belongs to the site owner and not the site) that has a lot of btc in it.