Generally speaking, lines are traditionally numbered based on whoever gives you the best chances at the highest goal differential.
Blackhawks never really did that. They defined lines by roles, and ice time was dictated by whichever role was most necessary. When the Blackhawks were trailing, the 2nd line got a ton of ice time because they were the scoring line. If you were leading for most of the game, then Toews and Kruger saw more time, and if you were on the road, Toews saw more time, vs at home, the Kane line and '3rd' line got more time cause Q could get them out more in favorable matchups, etc.
Whether or not those roles can still hold given the talent they have and, more specifically the TYPE of talent they have (a lot more one-way offensive players than two-way players as in years past), will have to be seen.
I generally refer to the 'top line' as whichever line goes up against the toughest competition and/or provides the best goal differential.