I'd like to make yogurt with non-fat dried milk enriched with canned (full-fat) evaporated milk. Since both these products have already been heated in the manufacture, do I need to heat/cool the mixture (as I would fresh milk) before adding the starter culture?
I would say, go with your gut. It is possible that you see a very slight advantage, or that you don't have any at all, so do as you feel better.
The denaturing of proteins has already been done in both the canned and dried milks, so you don't need to heat up for that part. What you can win is a bit on the contamination front. Making yogurt is about creating an ecosystem, and to give your preferred culture the best start in life, you are trying to keep the mixture as sterile as possible. The combination of canned milk, milk powder and water shouldn't have any more bacteria and yeasts than anything else in your kitchen, and heated milk gets re-contaminated during cooling, so if you can get any edge, it will be small. But if you want to shoot for it, there is no harm in that. Especially if you are reinoculating from the old batch a lot, this might reduce the time until you have to refresh your culture.