I found that when I am using the #57 foot, the one with the little vertical quarter-inch guide, when I let go of the fabric as the last bit disappears under the foot, it was tending to get gathered to the left, and I lost as much as an eighth of an inch of seam width. That was bugging the heck out of me, because of course if a seam is 1/4 inch wide at one end and 1/8th of an inch wide at the other, the edges are not going to be square when the patch is opened. I posted questions to a few quilting E-groups, and got various replies, ranging from "don't use that foot, use a wider one" to "Bernina has had that problem for years and never done anything about it". The most common response was to use a tool called a Stiletto that can go under the foot with the fabric and guide it for that last little bit.
I went to my dealer tonight and asked what they thought. They said to just use a thin, pointed piece of wood or something, like a shish-kebab skewer. I think I am going to use that most useful of all sewing tools, the seam-ripper. If I need to. I'm not sure I do. A couple days ago, I bought two yards of fabric expressly to practice with. I cut out a few 2 1/2-inch squares and sewed them together, and didn't have any problem.
The women at the quilt store were suitably impressed with the deal Cindy got for me. They said they still had a few 640s in stock, and they were trying to get rid of them for $4,000.
My next challenge is sewing over seams. If I plan carefully, I can greatly reduce the number of Y seams I need. In their place, I will sew two blocks together, and then sew a long side of the patch to a larger block, so the new seam is perpendicular to the old seam. If the seam allowance has been pressed toward me, so that the fold goes under the foot before the edge of the fabric, then I don't have a problem. The pieces line up nicely. But if the allowance is pressed away from me, the top block seems to get pushed forward, and the fabric bunches up toward the last pin. Or, if the pin has already been removed, the top piece ends up being pushed out of alignment. I haven't found a reliable solution for that yet. The last time I tried, I stopped the new seam just before I got to the edge of the fabric of the old seam, took the pieces out of the machine, and checked alignment. It was good. Then, I put the pieces back in, started sewing where I left off, and the results were aligned perfectly. So maybe that's a solution. In earlier attempts, just stopping with the needle down, raising the foot, making sure the seam is flat, and continuing the seam did not work very well.
Oh, one other thing. When I took the machine out of the case, plugged the cords in, and turned it on, it made a very scary grinding sound. It wasn't like gears that weren't meshing, but it sounded as though something had gotten knocked way out of alignment. I turned the machine off, took out the thread in case it was getting somewhere it shouldn't be, and turned it back on. Same thing. I looked more closely to see if I could see anything at all that was wrong. I saw that the guide that holds the bobbin thread in place as a bobbin is being wound had accidentally been pushed, turning on the bobbin winding motor.