I think that they are right now, and I think that their long and constant history makes it easier.  Even when they had their Pursuit Back Halves, the one being pursued was still drawn to the other.  It wasn’t like Puckleberry or Fuinn where, when they weren’t a couple, they weren’t anything at all.  Finn can never ignore Rachel, and Rachel can never ignore Finn—and with that history, I think they’ve arrived at the status of best friends this season.

I think they’ve always been friends, right from the start.  But I’m not sure about best friends.  While they could never fully ignore one another—nor want to—they weren’t completely honest with one another, and that’s pretty crucial for the best friend status.

They were more open during season 2, though.  Rachel made her feelings clear; Finn made them as clear as he could (he was just confused himself).  She went to him on advice, and so did he.

So maybe, sometime during season 2, they realized they were best friends, and that’s where the rekindling of their love came about and made them how stable they are this season.