I did it once when I got my mini 4 from my ex girlfriend (we swapped iPads, I gave her the air and she gave me the mini 4). But I have never created a separate Apple ID myself. What would be the advantage for betas?
I think at one time, Apple was making changes to Photos, Reminders, Notes, etc. on iCloud along with the new iOS firmware. Having a separate account means the beta won't affect or break compatibility of these services on my primary devices.