Years ago, I wove a blue and white "plaid" twill towel, with 1/2" or so of plain weave on each end, and hemstitched on the loom (above). I didn't recall any issue between the plain weave hems and the twill body of the towel. Both warp and weft were Swedish cottolin.
I recently read somewhere about tabby and twill together causing twill to ripple. When I began the first twill towel on this striped unbleached/bleached cottolin warp, that is exactly what happened. Ripples, probably more noticeable because of using different wefts for hem and body of the towel, and using one of them doubled. I had begun with a cottolin plain weave hem (to turn up and sew later), then changed weft to the linen/cotton slub, using a shuttle that holds two bobbins. I thought a bit coarser, rustic look would give variety to this series of towels, but felt the double weft would not make a nice, sewn hem.
Not pleased with what was happening, I decided this morning the beginning of this first twill towel was now a sample (above), as I needed to discover what was going to work best for hem and twill. The cottolin hem was woven so I wove a couple inches of twill, then wove another hem using a single strand of the linen/cotton slub, woven in some string and removed it from the loom.
Since I still had the sample (above) from the very beginning of the warp, and both samples had ripples, both were tossed into the washer (regular cycle, warm water) and drier (normal cycle), removing them while still slightly damp. When they came out, the ripples were more pronounced. When pressed with an iron, the ripples pressed out. Selvedges were still a bit of an issue where plain weave and twill changed. Edges were rotary cut, and photos taken.
Tomorrow morning, I will begin again, using a single strand of the linen/cotton slub for both hem and twill, which should alleviate some of the problem. I expect if I weave 1/2" or so hem and hemstitch on the loom, there would be little or no ripple effect. We'll see, on this towel, or the next.