If I understand the rage correctly yes, indeed: A 3GB file will download on your iDevice no matter what you do (no "disable automatic OTA update downloads" in the settings) and after that, you get the red "1" on the settings icon and can't delete this file even if you have no intention to install iOS 7. (As the file is "only" about 1GB big when you download it on iTunes, maybe the rest is "blocked space" on your iPad for installation?) The file won't download as long as you have too less space left.
That's how I understand the infos I get here.
EDIT:
Just saw this ... So "only" 750 MB and not 3GB? Where does the second number come from, is this the free space you need to have on your iDevice when finally choosing to install?
Correct, it will refuse to install if it doesn't have enough free space, just like any other software. 3GB is only "wasted" if you are actually installing the download, and much/all of it will be reclaimed after the install (I have no idea how much space iOS 6 or 7 takes up offhand, but it's probably similar enough to just say "all").