I would use a interfacing, whether a mid-weight iron-on or like Deco-bond. I used the prescribed Deco-bond in a "Professional tote" that I made and I would be very unhappy if I would not of added the interfacing, It would of been awful flimzy. I have a pattern to do this little wallet and my plans are to add the interfacing.
I only used Heat and Bond and Wonder-under for applique. Course now there is Steam-a-seam II, that is what I use for my applique work. Heat and Bond messes up my machine and needles.