IMO, I wouldn't say a box design tool is a MUST....helpful at times, yes. I haven't tried the big fancy programs, but any of the freebies I've used I found lacking for designing BP enclosures. They're pretty good for sealed and ported enclosures. As long as you like math you can always plot the formulas on paper.
I would take pencil and paper and good old fashioned experience over WinISD, etc. any day.