Hello Mahlon,
I tired to tab through the screen that has dropdown field, it seems to be working fine. Can you give me more detail or particular area where you want me to test it. Although I recently found a bug where on the New Invoice screen, if you search for a customer on the dropdown on New invoice screen who is not in your account and then you try for a customer in your account it does not show up.
I have already created a ticket for that. But regarding the Tab through the screen let me know if it is on a particular screen.
-Inder