The games individually go for cheap enough that i doubt a bundle would be saving much in the long run. With that being said, even if you pick them up now, I wouldn't recommend playing up through chapter 7 yet as while most of the other chapters are pretty "self contained", chapter 7/8 are meant to be played back to back for reasons I won't spoil.
Ooh. Thanks for the advice. Was also wondering about that. I've got 1 and 2 now, and I'll see if I can find a nice offer for the rest on trading boards. I'll hold off on 7 then.