There are option for use of Tags in the main Settings (the "gear" icon on the Dashboard).

In the Sales section, there's an on/off option under "Sales form content" header for Tags.

In the Expenses section, there's another on/off option under "Bills and Expenses" for "Show Tags field on expense and purchase forms".  

My only issue with the latter is that, at least in my setup, it's redundant to Tag both the PO's and the resulting Bills.