I thought it was the other way 'round -- that the loop should be tighter for modern indexed, because more loop equals more possibility for flexing of the housing, and you don't want flexing for indexing. On the other hand, "back in the day", when housing was often un-lined, and shifters on "good" bikes were usually on the downtube, a bigger loop made for smoother shifting, and since that loop was the only housing between the shifter and the derailleur, a couple more inches were no big deal.

That's the way I remember it, anyway. Seems like I remember reading it on Sheldon. I dunno.