I'd always heard the post-vowel thing, too, but it looks like that's wrong. According to a couple of greek pronunciation websites (see, for instance,
http://www.smartphrase.com/Greek/gr_pronunciation.shtml), it's supposed to be zee and fee all the time, because of the way iota gets pronounced. But given all the discussion that's out there on the topic (just do a search for "greek pronunciation" and you get deluged, and do check out
http://www.artsci.wustl.edu/%7Ecwcon...s/CompPhon.pdf for just a little idea of the complexities -- yawn!), I'm thinking this isn't something you can just resolve in a quick little answer on Greekchat.