Oh, well, duh. That was a stupid question, huh? Well now I am wondering why it's safe to use for water bathed jellies with Pomona's Pectin, but I'll just not get too off topic and keep it to myself.
On topic, I have done both- I have just plopped a roo in and babied them. I have found if he's the only roo going into a group of hens and he's fully developed, I don't need much introduction. It only seems to be a problem if he's especially frisky, there aren't enough hens, or he's a younger roo.
Even "mean" roosters generally aren't mean to hens unless there is another roo around in my flock. We typically only keep one per breed, but that might change when we're completely transitioned to one breed.