They are removable, but it's not easy or generally needed. You should remove the nipple (or flash hole liner on a flint) and clean and lube the threads. There will come a time when you need to to trickle in powder behind a bullet you seated without a charge or to kill a charge before you pull the bullet. Nipples will need replacing eventually, as will inline breech plugs. Shortcutting maintenance will get you in the end (or the eye!)
You can dunk if you want, but you certainly don't have too with 777 or BH, inline or sidelock. If you dismantle your rifle to clean it, do you really know it's still going to be sighted in when you put it back together?
