If I prewash the fabric (all the fabric) I don't wash the finished quilt but I will run it through the dryer with a wet bath towel and a softener sheet. That freshens up the quilt and gets all the threads off. It also frees all the threads that still need to be nipped.
The one I am working on now is not prewashed and it is going to someone battling cancer. I will wash all the chemicals out and send a freshly laundered quilt.
Sorry, but I do not understand the idea of seams coming apart. Don't you check the seams when you sew the pieces together? I usually pin at the intersections (to avoid flipped seam allowances) and I pin on long stretches so nothing will slip. I have never had an open seam or barely there seam. If that is the case I rip and resew. In the last quilt I had one square that was too small but when I adjusted, I still had over 1/8" seam on the side.