Very old-school, prim and proper types think it's tacky. (Think 85-year-old, stuffy librarian-type stationary store owner. You know, the one who turns up her nose when you tell her you want thermography rather than real engraving... Yeah. Her.)
FYI, they also think it's tacky to include RSVP's. (Just ask Miss Manners. She'll tell you how tacky it is!

)
They're from "back in the day" when each invitation was answered with a handwritten acceptance or declination. Can you imagine? Well, I can. I got 2 when I got married 10 years ago, both from older, rather proper ladies. I love it! And while I admire that level of commitment to proper rules of civility, I choose to move on. We don't all carry around calling cards any more, do we? Well, I know I don't!
We live in the here-and-now when some folks are starting to email invites. SO, I say live in the present and include your directions with the invite. It makes sense to keep it all together...that way your guests aren't waiting for directions, or possibly missed during a second mailing.
Best wishes to the bride and groom!