For me, the stem is dependent on the top tube and handlebar in order to achieve the desired fit. If the handlebar happens to have a 25.4mm clamp section I'd prefer to have a stem with a 25.4mm clamp, but if I don't easily find one I have no qualms using a shim. The brand (group), angle and finish are all things I consider. I have used both the Nitto and VO shims and they work as advertised and are plenty stable. You do need to take care when installing them not to scratch the engraving on your fancy handlebar, though.