as an example i used to do flat bench presses like a powerlifter with an arch in my back and my butt barely touching the bench. i could definately handle more weight but my pecs improved drastically when i started benching with my feet up and totally flat against the bench. i don't bench at all now due to years of shoulder problems but my goal even on compound movements is to isolate the target muscle by eliminating momentum and leverage from support muscles. i hope this helps.
That is exactly what I said over at OLM and I got so much shit for it because some of the guys over there believe that you HAVE to bench the dave tate way.
Your back should naturally arch if you have your shoulders in the lowest position to achieve maximum pectoral involvement; as in shoulder blades clenched together and your shoulders pulled down towards your body. Now, if you mean your back arches because your driving with your feet and turning it into a powerlifting movement, then that's more of a technique to lift more weight rather than focusing on the targeted muscle.