I put all my decals on dry. If you get a big decal like a hood blackout I would do it wet. I have a vinyl cutting plotter and make my own decals and have been making signs, banners and lettering tow trucks for over 10 years now on the side. With all the decals I've installed I've never used the wet method but I will be trying it when I do my hood blackout.
2003 Jeep TJ Silver 4.0L, Auto, 5.13 G2 D44's on 35's
1998 Jeep TJ Sahara Black 4.0L 5-Speed, Replacing the frame
1997 "GI" Jeep TJ OD Green 2.5L, 5-Speed, lifted on 33's