It depends on alot of things. unless ur going all out or can get a real good deal by a professional with a flow bench, Id say dont get it. Alot of heads now a days are pretty efficient from the factory. In most cases you will not feel any difference from a butt-dyno.
Alot of machinists have different techniques and theories out of getting maximum efficiency. If you are still interested then here are some good reads that I found year back when was doing my head:
Head terminology:
http://www.alaniztechnologies.com/headterms.html
Very good write-up:
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=4
Dyi guide: