As it turned out, of course, the western allies' fears about Stalin making a separate peace with Hitler were baseless.
Rather, it was Stalin who feared Britain and USA might be the ones to make peace -- and then might join with Germany in renewed attacks against Russia. Given what Russia had suffered at Germany's hands, and how superior Russia was militarily by mid-1944 (when they launched Bagration, the Really Big Operation), there was no way Stalin was going to offer terms to Germany.
Still, history must regard Dieppe as decisive, for the effect it had on planning for D-day.