it helps...but is not necessary.
There are guys running around with out a tune and just a cam swap and haven't really had any problems.
I started with a cam swap, so didnt plan on a tune, and ended up with porting heads/intakes and removing some things form the engine so I kinda need one now.
1998 Subaru Legacy L Sedan
1999 V6 5-speed
14.6 @ 95 w/ 2.22 60'