You can turn on the sales tax feature in AP, but I'm not a big fan of it.  I would say it depends on what level of tracking you require.  If you want/need the ability to show you paid tax to different states or taxing jurisdictions like you do with sales orders, you'll want to use the sales tax feature built into the software.  If you just want to expense the sales tax to a certain GL account and have a PO that matches your vendor invoice to the penny, just create a miscellaneous charge code (i.e. "/SALESTAX") and add it to each PO when you create it, or add it in during the receipt of invoice process.  That's what we do.