Websites not only don't need to look the same in all browsers, the should not look the same. It is impossible to have everything 100% identical in every screen resolution, every OS, every device, and every browser and their respective versions. The thought alone is absurd. We shouldn't (and can't) expect a site to look the same on a screen as large as a TV as it would on a mobile phone. Likewise, it'd be absurd - perhaps even impossible - to serve each device their own stylesheet.
With that said, you don't need to include that CSS3 fix for IE. It's nice having it if you feel that the features add to the experience. If it doesn't, you could as well save the extra bytes and just let IE degrade gracefully and have these users see square corners instead.
I take the view that it makes for good practice to use the lightest, most efficient and sensible coding techniques that serve the majority of users. The rest of users are served a functional, aesthetically pleasing site without the extra fine touches.