In my opinion, CS5 does a superb job stitching and merging. The success of the stitch depends on the elements that form the entire picture. Sometimes CS5 renders a perfect stitch when everything "helped". In other pictures, what seems to be a perfect stitch, is not. If you look closer at 200% is possible you will find minute errors. Errors are more prone to appear in interior pictures with intrinsic lines. One of the worst scenarios is an interior with shape designs on the floor in combination with ceilings with beams not shot "squarely". I've found that CS5 fails every time under these circumstances.
I also noticed that every time you feed the same pictures to Photomerge, the program does a slightly different cut-off tiling. I don't know why. Maybe that's what software engineers call fuzzy logic.
In my case, if I want to make sure I'll end with a perfect stitch, I use my slider. If I was too lazy to set it up, in a hurry or left it home, I always check at 200 or 300% after using Photomerge. If I find errors, well, it's time for some cloning and/or some copy&paste + move/rotate. At times, it can be really tedious. Every now and then, I just press the "Ignore Key".
I made my "flat-stitch slide ruler" out of a discarded Minolta Close-Up bellows for almost nothing. I'd love to have the Hartblei Collar for the convenience and speed but it is too pricy for me.
Eduardo
P.S. Or perhaps Kirk, you should share your workflow.
Here is the main issue. Why do you need it? I have made a large part of my income daily shooting architecture and interiors with flat stitches from Cannon T/S lenses a Canon 5dII and PS CS5 and never ever worry about parallax problems.