For example, Google.com 
The SOA email is : dns-admin.google.com
The contact is:
Administrative Contact: DNS Admin Google Inc. dns-admin.google.com
As you can see, both are using
google.com, I am thinking it is safe to use the same domain, i.e. consider the case you lost control of the domain, you can receive email also.
(Of course Google is a public company so the chance is low, but might occur for smaller company that their domain might be stolen..)
So, do you recommend use your the same domain as the contact or others free services such as gmail?