It helps get the perfectest bore dimensions, but I've built a few redblocks and never used one and not had any (apparent) issues with ring sealing/compression. I have done a B230FT from STD to second overbore that way, no issues.
Still, if money was no object, or I was chasing 300hp+, I would probably buy one, after a certain point I think the potential cost of failure becomes high enough that it's worth the extra spend.