Is salt important to the texture of dough?
Salt toughens gluten and makes the dough less sticky.  So with that said, it would make it much easier to work in too much flour and make the pierogi dough more dense.  Do a search on the following page for "biscuit" and read the few paragraphs before.  

http://www.saltinstitute.org/Articles-references/References-on-salt-use/References-on-salt-use/References-on-salt-in-food