For the purposes of the GRE, if we're rounding to the nearest whole number, we round .00 to .49 down and .50 to .99 up.
What's more, we typically only look at the next digit over, so if we are rounding 3.48 to the units digit, then we'd look at it as 3.4 which rounds to 3. We don't carry the rounding past one digit, for example 3.48 to 3.5 to 4. Instead, 3.48 rounds down to 3.
This is just one convention of many, but it's what the GRE uses, so we follow that. Wikipedia actually has quite a long entry on rounding if you're interested in the full history and conventions: http://en.wikipedia.org/wiki/Rounding
Our own Mike McGarry wrote a fantastic blog article on this, too--check that out for a more detailed explanation.
And finally, the end of this video lesson (around 6:30) covers rounding in more detail.