There are many do and don'ts in playwriting and quite likely the first and most important rule should be to disregard all rules. That said, I am about to make a very strong recommendation, formed after a lifetime of watching theatre, writing for it, and teaching others... Leave dreams out of it. Think about it. When someone starts telling you about that dream they had last night - do you get all excited and big-eared, or do you start to zone out? I am guessing that 90% of you do the latter. And from a writing standpoint: having a character recount a dream is almost invariably a (lazy) way for the writer to announce his play's Big Themes. Often in uncharacteristically elegant language. Don't do it. Show don't tell. And leave the dreams to the poets. \
Dave Carley is a Playwright with over twenty years experience and 400 productions and yes, has used dreams in plays but knows better. Check out Dave's website at http://www.davecarley.com