The biggies like WM, AAP, AZ, O'R never listed in participating retailers. That said, last time purchased from WM and got my rebate. I would assume nothing has changed. Go to the slickdeals site now and read same thing, WM included.
I'd add that unlike SOPUS, for those that are more comfortable doing it old school, XOM does include an official printable rebate form
on thier site. That's the way I did it last and received my rebate in the promised/specified time frame. Kudos to XOM.
Seeing as I don't need it currently, before pulling the trigger likely wait and see if M1 goes on rollback a few bucks. Even without rb, a good deal