I think SharePoint 2013 is recent enough that there's no best practice yet, but as with any upgrade, I would recommend testing if possible. If you have a spare server, where you can upgrade from MOSS 2007 to SharePoint 2013, it would be a good test. If you don't have a place to test, then make sure to have a backup. Back up all your form templates, your XML forms, etc. And perhaps most important of all, communicate with users, to make sure they understand the upgrade and the possibility of downtime as issues are worked out.
InfoPath 2007 and 2010 forms, in theory, should work in SharePoint 2013, but it's always good to test.