Personally I think it depends on the site. On a site like Apple's with the menu stretching the whole width then I think equal size is the best approach. If the menu isn't stretching the whole width then I would probably go for variable lengths
I would say each menu item same width as a general rule, but having said that if the menu items are of similar width (text) then the width of the item looks fine, just if there's a real contrast with one really long & one short it can look bad IMO. Hope that makes sense/helps in some way!