Some teams stack the first line and rely on them to do the heavy lifting. Ideally a team has two lines capable of scoring and exploiting 2nd pairing defenders. You know, finding an opening when Suter finally needs oxygen.
The Penguins have had so much success because they have two first line centers. Crosby is technically your "1st line" guy, but not every year. Their ice time is going to be pretty similar.
So to answer your question, it depends. The top center should be on the ice with his line the most shifts possible. A lot of it is matchups. Zetterberg spent two playoffs shadowing Crosby. The entire plan for the Wings was never let Crosby move without Z near him. In later years we saw Glendening shadowing top lines against Tampa.
this is why I lean towards how @tsweeney talked about it above. Your first line guy needs to be a guy you are willing to put out there against anyone, in any circumstance, and feel pretty good about. It's what made Z our #1 center (yeah, I said it) and what should have told us how horribly screwed we were when the coaches decided that matching Glendening up against the other team's top players was a necessity. It's part of what signaled the end for Lewis when he thought he had to chase Iginla with Draper getting 20+ minutes a night.
It becomes a bit more grey on the wings where how a guy fits into a particular line can be more important than his "skill level" but the guy still can't be a plug and play 18 minutes a night on your first line.