The author of any recipe which specifies the kind of oatmeal to use thinks so. On the Quaker Oats site, for example, all recipes specify which product to use or if either is acceptable.
In general, if a recipe specifies which kind of oats to use, I’d follow the recipe. Each will give a slightly different quality to the cookies, although sometimes it’s minor enough not to matter. A look at how rolled oats differ from quick oats will explain why.
They both begin as oat groats, whole hulled oat grains, which take a good hour or more to cook. Steel-cut oats (also called pinhead or Irish Oats) are groats cut into thirds, which cuts the cooking time to about 30 minutes. Some cookie recipes call for steel-cut oats, although they’re less common.
Another method to reduce the cooking time of oats is to flatten or roll them. Rolled oats will cook in three to five minutes after coming to a boil, rather than half an hour. Taken one step further, quick-cooking oats take just one minute to cook because they have been cut into pieces and rolled a little thinner than regular rolled oats.
What does this mean to the baker? Nutrition and taste aside, oats are used to give crunch and texture to cookies. If maximizing crunch is important, use rolled oats; they give a more substantial texture because the oats are larger. Your cookies will be slightly drier and chewy. Drop cookies made with them will spread out more in baking. Quick-cooking oats will yield a slightly flatter cookie than old-fashioned rolled oats.
If you bake more than one kind of oatmeal cookies, you may not mind stocking both in your pantry. If all you have on hand is one kind, you can substitute one for the other as long as you don’t mind a slight change of texture. If you really don’t want to take any chances, here are some tips for ensuring a successful substitution:
If substituting rolled oats for quick oats in a cookie recipe, consider (1) refrigerating the dough for 20 minutes before baking, (2) adding 1 to 2 tablespoons additional flour to the amount called for in the recipe, or (3) if you want to dirty up something else in the kitchen, you can pulse the rolled oats in the food processor briefly (one to two pulses) to approximate the consistency of quick oats. If substituting quick oats for rolled, you can try reducing any liquid called for by 1 to 2 tablespoons.