If you attempt to you will get message like:
You are using Desktop version: 6.1 and Server version: 6.0. To upgrade to compatible versions of Desktop and Server please contact Tableau Customer Service at firstname.lastname@example.org or call (206) 633-3400 x3.
So the answer is, Desktop 6.0 can publish to Server 6.1, but Desktop 6.1 cannot publish to Server 6.0
Thanks Joe. That was my suspicion, but I wanted to confirm.