Actually, using track motion for stills is generally not a good idea. The reason? Track motion works on the final video, not on the original source file.
For flipping, it probably doesn't matter. However, if you are tempted to zoom while you are in the track control, you will zoom in on a 720x480 (or for Grazie, the PAL equivalent) representation of your still. If your still is 3000x2000 (to take an extreme example), that will first be subsampled to 720x480 (or whatever) and then that will be zoomed. By contrast, if you do the zoom in the pan crop, Vegas will zoom in on the 3000x2000 original and then sample that original to come up with the zoom. Obviously, the difference in quality will be significantly better with the pan/crop than with the track motion. Thus, if you do a 2x zoom on this hypothetical 3000x2000 image, the track motion will give you a yucky (technical term) 360x240 resolution, whereas pan/crop will give you a pristine 720x480 (or whatever the project setting is) final resolution.
Remember, this only applies to situations where the source media is higher resolution than the project settings. If you are simply using video that is the same resolution as the project, then go ahead and use track motion for everything, if you like.