When I am finishing my handwoven towels, I machine zigzag on either side of a separator pick between each towel, cut the towels apart, and throw them in a hot washing machine and hot dryer. I then press each towel, turning under and pinning a hem as I press. I finally finish the hem with a machine straight stitch. I know some weavers hem their towels before finishing in the washing machine and dryer. Do you have a preferred way?
The only difference in terms of consequence between the two methods (cutting but not hemming vs sewing the hems before washing) that I can think of is that if they are hemmed before washing, shrinkage might occur less in the hem area because of the added support of the stitching, widthwise. In this case, the hem might splay out a little more than the body of the towel. For this reason, I would prefer to do as you do, except that I probably would not cut apart the fabric until after washing the whole piece unless the piece is really, really long. For more than three yards, say, I might divide it into sections (that I'd zigzag as above). Not cutting the towels apart first allows skipping the machine zigzagging except on the ends and also avoids any raveling along the zigzagged edge (which can happen, depending on the density of the towel weave structure). The only reason I wouldn't wash a very long piece is possible distortion from twisting and knotting in my top loading washer (not that I've ever had that happen).