Depends on what engine you are tuning for.

If you want your torque peak at 7500 and hp peak at 9500, then equal lengths become more critical, and the lengths need to calculated accordingly.

But for the most of us, if the fronts end up a little longer, and the rears end up a little shorter than a "calculated" torque peak of say 5000 rpm, the peak will be a little less, but the curve will be flatter. So some more at the bottom end, and a little more at the top.

One of my cars lost top end, when going from tri-Ys to equal 4 into 1s, both sets 1 3/4 inch primary pipe.

Here is one site I like for the construction info they have:

http://www.stainlessheaders.com/
Gary