I have recently been encountering these fraction keyframes recently at work and they have been a **** to solve. you can't snap them to whole frames if there is a set key already on it. even when i set the graph editor and timeline to only key on integers of 1, they still appear. I did come across as to the reason why they started appearing. Here's a scenario:
Say I have key poses of a character set on Frame 5 and 15. I don't like the pose of the head tilt at frame 5 so I scrub through the timeline to go 5 but I accidentally stop at 7 and set a key on the head adjustment. I'm like "oh shoot, i need to put the keyframe from 7 to 5" so I (red) highlight the key on 7 and click drag it to 5 and set it. This maneuver created the fraction keys cause Maya sees frame 5 as already occupied so it shifts the key from 7 to something like 5.032.
Here's fast way to delete the fraction key frames that are already in your scene instead of digging through the graph editor:
Select all controls that are keyed, use the [<] or [>] keys to scrub through each keyframe in your time line till you see a frame number with a ".00" after it. right click that set of keys (making sure its "5.00" not "5" ) and delete the frames.
To prevent the fraction frames from occurring if you misplaced a key frame on the wrong frame number, simply:
go to the key you want to middle mouse click + drag it on the timeline to the frame that has the old key and set it. it should replace the key on the whole number.
Hope this one specific scenario helps understand the fraction key issue.