Eyonix told us all when 3.0.2 was released that we'll need to keep the patch files we get. If you delete the patch files you'll need to redownload everything you got with patch 3.0.2 three weeks ago, and patch 3.0.3 which you'll get today.
That's a lot of downloading, and potentially on clogged servers around the internet as millions of people rush to play Wrath of the Lich King as soon as it's released. Further, do not count on mirrors to be working when Wrath hits. Chances are they will be overloaded as well.
The best thing you can do is to take all your 3.x patch files later tonight and copy them to a folder tucked safely away. Preferably someplace where your kid(s)/girlfriend(s)/boyfriend(s)/parent(s) can't get at them.