It would seem to be a mater of what pleases the eye. Compare hand change thicknesses for heft and appeal. Whatever looks good is good. Limitations would be what you can bend or forge into the shape of the tube. Also if you intend to braze it to the bike, keep in mind the titanic amount of heat that could be required to get it attached to a possibly thin tube. That and the desire to paint under the badge may account for why people pin them on in some cases. Same issue if you are brazing them together. While you can play with various issues around braze melting temps and remelting temps (often higher), A really thick piece and different layers may make for tricky final brazing.