I think that if Wine has been compiled with wayland support, that is should be opt out at run time. Of course if the user is running pure X11, the application should fallback to that backend instead.
At least to me this makes most sense as people running Wayland would most likely want to use the native backend if it is available. At least to me, opt in is very annoying as most other programs I use are more buggy with Xwayland than with their native Wayland backend. This also ensures that it gets the most testing as early as possible.