Most of the time it's fine. I wouldn't mix really docile breeds with really aggressive breeds. If you do, keep an eye on them, even if they were raised together.
When I first got chickens, my two Dorking pullets died from injuries they received from what I purchased as Araucanas. Apparently they were mixed and half B%&!#@. I pulled the Dorkings immediately and treated with antibiotics, but it didn't matter. They still died.
These birds were raised together as chicks. They had over 5 square feet per chicken in the coop and free ranged over several acres during the day. The attack happened after they were full size, although I don't remember the exact age.
I know it doesn't happen often, but there's a reason that mixed flock aggression is mentioned in books on chickens as a possibility. I think it's fantastic that so many people here have had no problem at all. I wish that had been my experience, too.