I've been buying pre-packaged pierogi from Trader Joe's and local Polish delis. But I can't seem to get a straight or consistent answer on how you are supposed to cook them. Should you steam pierogi, boil them, or saute them with butter? 
My babcia (grandmother, she came to the states in '49) boils them after assembly. If she's serving them right away, she browns them in some butter. Otherwise, she packages them up in ziplock bags and freezes them.  Packaged pierogi are probably already boiled. I would thaw them and saute in a little butter until golden brown and warm through. 