Regarding wet-finishing handwoven towels before or after hemming, here is my rationale for doing hemming after washing.
I always wet-finish before touching my fabric with an iron for the following reasons. Wet finishing removes the spinning oils, allows shrinkage to take place and also fills in the spaces between the threads when the yarn softens with the washing.
Even with washing first, some reed lines may still be faintly visible, as the center towel shows.
Part of the wet finishing process is finishing your cloth with a hard press and that is not the same thing as ironing. Ironing is gliding the iron over the surface of the cloth. Hard pressing is exactly that--pressing hard for several seconds without gliding the iron.
What the hard press or cold mangling does is set the threads in the fabric, creating a memory for those threads. It makes it a little bit harder for the fabric to unravel and move in the weave. By pressing a hem in first before wet finishing, irregularities such as reed marks that are there before wet finishing will be made permanent. Those irregularities become the memory of the fabric and more than likely will not wash out with wet finishing later.
I sometimes wash all the towels I've made without separating them, but more than likely I will separate them with the serger because I have used colors that I wouldn't want to bleed onto another towel. I try to use dye fast yarns, but occasionally one isn't as fast as was claimed by the seller. I hate those kinds of surprises!
This is an example of before and after washing. Notice how the purple bled, changing the white yarn into a lighter purple. If I had not cut the three towels apart and washed them separately, the small sample at the end would have ruined the two other towels.
Another reason to separate a long row of towels is that they can get
twisted in the washer and dryer and that will often permanently set
wrinkles into the fabric.