For what it's worth, most of the apostrophe/single quotations marks end up being question marks in the mails I receive. Single straight ones come through fine. Not sure how it's for others. I'm using Outlook/Exchange here, getting the digest mails from the list. For example a few lines from one of Lievens mails:

It doesn't only look very ugly when you read a text like {QI?tu? ?och vI?el?a?} where the quotes jump back and forth, but it's also just the (In case it turns back to apostrophes, the klingon has ? every third character.)

Personally, I only use the ', mainly because it's easier to type - the others require a press of the spacebar to appear. (Or another character that can't be modified by them.) Christoph