Okay, I'm a sneak and would probably just do it, but probably asking would be better. If they don't have a strict dress code, you'd probably get away with it.
I worked for a place where I was worried they might freak about dyed hair and covered over my dark red when job hunting. Much to my surprise, there were people there who had just been hired who had piercings and the like. Oh well...
If you're bold, braid it and see if anyone says anything. Braids can be easily taken out. If they'll freak about dreads, they'll freak about "ethnic" looking braids, I think.
As long as you keep the dreads nice, I can't see why anyone would care. I've seen some really nice looking dreads. You have really long hair, so I think you could pull it off. It's different than someone who'll look like they rolled out of bed after a long night of doobin' and Dorito eatin'...LOL...