the only reason to blanch a vegetable is because it makes it softer and easier to eat however it does cause it to break down faster in the water. If you put the vegetable in and after a couple times(so they realize it's food) you notice it takes a while for them to start eating it, basically they wait for it to get soft, then that would be a good reason to blanch. Harder foods like tubers and carrots are an example of foods that should be blanched. Basically you need to tailor the food to the individual pleco's preference. It's just like with bristlenose fry you need to use french cut green beans because with regular cut green beans they have a hard time with the skin. If his pleco wasn't already 6" I could see blanching the cucumber because the skin would be hard to eat.
As far as wood eating yes only a small percentage of plecos mainly eat wood, like the Panaque and Panaqolus, but a large percentage do rasp the decaying biofilm on the wood which they do eat some of the wood in the process and that contains cellulose
