There are several points at which it is a good idea to check.
Md5sum the ISO download. Why? Because it may have been corrupted in transit. While bittorrent is claimed to reduce chance of corruption, do take the time to compute the sum, it will only take less than a minute and save much pain later.
Verify after the burn. Why? You might have a faulty CD/DVD or drive.
Verify by running the media check. Why? This seems redundant to number 2. But if you are installing on a different machine, then note that you are using a different drive!
If you are installing directly from the ISO image via NFS, SMB or local file, then there’s no number 2, but 3 is still a good idea. You don’t need to redo 1 if you have a good md5sum for the ISO image file.
I’ve had problems with the 64 bit version of the install too. It installed 6 or 7 times (Courtesy of AsusTek and one of their dodgy Mobos) and then started throwing errors. I put the DVD in a cleaner and it works OK again. Guess it was just a dust-bug.