Jeff,
No, not using "save as" in P/M 5 until I've completed all the editing. Before I even upload P/M 5, I use the command (in aperture) "edit with external editor" up pops P/M 5 with the JPeg image I intend to edit, along with a Tiff copy of it.
Let's take PM out of the equation then. Look at the folder where you have exported the image. Do two files exist, one being a JPEG and one being a TIFF? It is extremely likely that Aperture is creating both files. I guarantee you that PM is not creating either file.
If you can't get Aperture to 'edit with external editor' without launching PM, temporarily delete PM 5 from your system (you can add it back in later) and Aperture should create both files and then fail to launch the missing PM 5. You should be able then to see both files in the destination folder with the Finder.
I'm very certain that the problem lies with Aperture and not with Photo Mechanic. Like I said, Photo Mechanic does not spontaneously create TIFF files.
-Kirk