I have a machine that has Office 2016 64 bit version, and we had SP Designer 32 bit running without issue. We needed to load Visio on this machine and only have access to the 64 bit version due to our 2016 Office Suite. So we upgraded the SP Designer to 64 bit and now we can't open 2013 SharePoint Designer workflows, but we can open 2010 workflows.
My personal machine is setup with all 64 bit and works just fine. And I had the same scenario a few months back that I needed Visio, so I upgraded from 32 to 64 bit.