You can sign into the wallet with any of your keys, have you tried it? And as always, your active will be used to make transactions. :)

There is a suggestion in the box to log in with your "posting" key, I assume to get users in the habit of using it. Then when you go to complete a transaction that requires a higher key, it prompts you to do so.

I just went through the whole process to walk through it.. seems pretty straight forward, just something new we have to get used to.