Often while cooking pasta I will take out some pasta water to incorporate into my sauce. The purpose is to transfer starches into the sauce to thicken and bind it.
Is it better to repeatedly skim the foam that typically forms on top of the pasta water for this purpose (assuming it is more starchy), or does it not make a difference?
Yes, that foam is mostly starch. However, for traditional pasta sauces you don't want only that foam for your sauce. You're emulsifying the sauce with starch and water, so you want at least some of the pasta water too.
Theoretically, by taking the foam and adding a little of the pasta water you'd end up adding a mixture that was starchier than just a scoop of the pasta water. I'm dubious that the difference is measureable, though; it takes very little starch to create a foam.
Is it better to repeatedly skim the foam that typically forms on top of the pasta water for this purpose (assuming it is more starchy), or does it not make a difference?
Yes, the starchiness of the "pasta foam" will be more concentrated than the "pasta water" itself. However, whether this is a good or bad thing will depend upon the application/recipe.
A recipe may be assuming specific starch (and salt) concentrations based upon previous measurements of the total pasta and water boiled. In this case, "add 100mL of pasta water" would not be the same as "add 20mL of skimmed pasta foam" because (even if the starch and salt levels were matched) the added water content is off-recipe. That may give you more control/flexibility in being able to use less water or add more later... but it may also just be overcomplicating the process.
That being said, the various potential avenues for inconsistency (used excess water to boil the pasta, used a particularly starchy/unstarchy pasta, etc.) can be a bit of a headache when control and consistency are important. So it is not uncommon for some chefs/restaurants/cooks to pre-make batches of "pasta water" (or a similar such slurry). This allows finer control and consistency of the exact amount of "starchiness" and "saltiness" as will be added from plate to plate on day to day.
