Not in this recipe. This is done mostly for aesthetic reasons since the gray foam looks unappetizing and will make the liquid in the pot unclear and cloudy. Since this recipe uses Guinness though, it is already dark and you don’t need to care about a clear broth. Boiling can actually turn your corned beef tough so it's best to keep it at a simmer and skip it. That being said, if you notice a lot of foam forming in the pot and you want to skim it off, of course you can go ahead and do so.