A Cre8asite Forums thread named #8 with appostraphe, not listed without discusses one members dilemma with using an apostrophe. He says, "I'm number 8 when I use "men's [plus my phrase]" on yahoo.com search" but when someone searches without the apostrophe, he is no where to be found.
What does he do?
(1) Keyword research; see which phrase is searches on more. I am not 100% sure if there are tools that differentiate between an apostrophe or not. If not... (2) Look at your statistics. Do a PPC campaign and try out both (um do PPC campaigns differentiate between an apostrophe or not, even with exact match?). (3) If not, do it the slow way and try to change it and target it without the apostrophe. Then wait and pump links to the page.
Overall you need to test to see if conversions and sales increase. If they do, then you know which way to go.
Discuss over at Cre8asite Forums.