This is both a problem and an answer I found to it, that I wanted to share.
As you know, Pro/E's 3D display comes with a "Spin Center", whose icon looks like a red, green, and blue ball tied together with pins. At our plant, the default is set to "Spin Center" ON, and the Spin Center is set to the Model Center.
With Spin Center ON, when you middle-mouse-button-click and move the mouse around, the model or assembly rotates about the default Spin Center. With Spin Center OFF, when you MMB-rotate, the model rotates around (a) some random location that Pro/E chose, or (b) whatever the last model feature was that you clicked on.
If you want to zero in on a specific part of your assembly for a moment, you can toggle Spin Center OFF using the Spin Center icon (which should be) at the top of your screen. Then click on one of the parts in the area you want to examine, then *click on it again* to select a vertex, edge, etc. Once this vertex, edge, etc is selected, subsequent MMB-rotates will cause the assembly to rotate about this point.
The catch is, as soon as you (often inadvertently) select some other sub-feature elsewhere, the assembly will then rotate around that point instead. So your center of rotation can change on a whim.
I had a problem with a large (physical size) assembly whose Model Center was for some reason way off in space. With Spin Center on, when I MMB-rotated it, it wallowed about in space most awkwardly. But with Spin Center off, while it would "behave itself" for a few moments, the center of rotation wouldn't stay put, and this was also a pain.
Finally I noticed that there was a stray DTM plane way off in space far from the rest of the assembly, and the Spin Center was set halfway between this stray plane and the assembly itself. (It took me this long to notice because normally I turn the display of the DTMs off immediately, else the assembly looks like a big brown hairball and I can't see anything.)
The stray DTM was the "TOP" assembly plane of a subassembly, which for some reason was located 1700 mm away from it, instead of having it lined up on the face of one of its parts, for example. As soon as I moved the subassembly to be right next to all its assembly planes and regenerated it, the Model Center and thus the Spin Center of my main assembly moved to inside the main assembly, and it MMB-rotated much more conveniently with Spin Center 'ON'.
Hope this helps.