Well all I know is that what they told me.
I'm thinking that person was either misinformed or just didn't know what they were talking about. I've had quite a few data transactions that were less than 1MB, and they come up as the correct amount, not rounded up.
As for the data usage issues some people are having, I have no clue what that is, because I haven't had any issues. I just have the 200MB plan, and in the last billing cycle (new one just started) I didn't even use half that 200MB (though that was over only 13 days I believe). Granted I am on Wi-Fi a lot when I'm at home or visiting my parent's house, and I make sure to do most of my large data transactions (most downloads, Pandora, YouTube,etc...) when on Wi-Fi. However, I have a number of things setup to automatically update at regular intervals, and did so before I found out about turning to the WiFi to never sleep, and even when the WiFi was sleeping with the screen off, I didn't use a ton of data.
One thing that may have helped, I have JuiceDefender on my phone to help with battery life a little, and one of the things it does is while the screen is off, it turns the cellular data connection off, and only turns it on for 1 minute every 15 mins to download updates and such. I'm not sure if that would make much of a difference, but it possibly could.