Silly question here but:  Does the unique MyDomain qualify for both production and sandbox instances? I would think so, but just want to double check out there.

So I could:

Register "crazydomain" in the sandbox and have:

https://crazydomain--crazydev.cs3.my.salesforce.com

And register "crazydomain" in Production and have:

https://crazydomain.my.salesforce.com 

(of course, I'd have to reserve/register in both places)