I see three distinctions, personally:
1. Acting #1: A 1st line center by default for nothing other than the fact they play on the teams first line. Skill level is irrelevant.
2. Legitimate #1 center: A 1st line center who would be one on any team discounting their own 1st line centers who may be more skilled. Must possess the skill level you would expect of a 1C, something that is highly debatable. (Stepan goes here.)
3. Franchise/Star #1 center: A 1st line center who could be the #1 center on even the best teams and plays an elite game at least offensively. Has an elite level of skill, considered a perennial all-star, could win the Art Ross/Richard any year in their prime. (Crosby, Getzlaf, Thornton, Stamkos, Malkin, Kopitar, Giroux etc.)
I never see Stepan getting to #3. He is definitely a 1st line center all things considered, however.