Me, personally, I took my passport photos and trimmed them down a tiny bit on all sides in order to make them the correct dimensions specified in the checklist. I was very careful to try my hardest to make them straight, to make them all match, and to not look like they had been cut. It was a pain in the behind and I bet the IO doesn't even care what size they are ... but I didn't want to risk it. Who knows, maybe they do take a ruler to your photos - lol!
LOL, I will forever have a vision of IO's sitting at their desk with rulers, while application files pile up all around them
We found a photographer that took the photos and cut them down for us (took him 2 minutes with a very old paper cutter). Would have taken us alot of stress and a few wasted photos to get it right I'm sure. As long as the interior measurements for position of head, background, etc., cutting down the photos, should be fine. That is probably the one thing that I haven't seen rejected since being on the forums, photos.