Correction:
I have tried this on a test machine that runs multiple OSes and FCPX versions for troubleshooting purposes. When I first did the test the machine was running 10.0.4 on Snow Leopard, and clip skimming was enabled. As I said these were the results:
When the Viewer is set to any zoom level, previewing an effect or skimming over a clip (if clip skimming is enabled) makes the zoom level temporarily jump to 100%. That is not a correct behavior, and here is a little video showing this issue:
files.me.com/ronnycourtens/u6wftl.mov
I have switched the test machine to run 10.0.4 on Lion (7.3) for a new test and these are the results:
When the Viewer is set to any zoom level, previewing an effect or skimming over a clip (if clip skimming is enabled) makes the zoom level temporarily jump to Fit. That's the behavior we are used to see and which I like.
This also explains why if the Viewer zoom level is already set to Fit and you preview an effect (or you skim a clip with clip skimming enabled) you won't notice any difference.