The Answer: it depends.
Freshly cooked pasta that’s going to be used right away in a sauce SHOULD NOT be rinsed (e.g. spaghetti in a meat sauce). Rinsing the pasta removes the starchy coating that helps the sauce stick to pasta.
The purpose of rinsing pasta is to stop it from cooking any further – think of it as hitting the pause button. That’s why it’s important to use cold water when rinsing cooked pasta.
The only time you need to rinse pasta is when you want to stop it from cooking further.
Pasta in Soup
Noodles used in soup are usually cooked and cooled ahead of time. If you were to cook the noodles in the soup, they usually over cook and turn to mush – yuck! Instead, the cooked and cooled noodles get added to the hot broth just before serving. That’s why when you grab a takeout order of pho, the noodles are served on the side, so they keep their just-cooked tenderness.
Another instance where pasta should be rinsed (again, under cold water) is when the noodles will be cooked further in the oven – say for lasagna or any other type of baked pasta dish. If you were to cook pasta noodles without rinsing under cold water and toss or layer with sauce then bake in the oven, the noodles would end up soft and gummy. By interrupting the cooking process, the noodles are less likely to overcook in the oven.
Pasta salad is another great example of a dish where cooked pasta needs to be rinsed under cold water. Cold water chills the noodles quickly, making them ready to be dressed. Since pasta salad isn’t cooked any further, slightly overcooking the noodles will give the salad the right texture when it firms up.
Hope this helps!