You should test for IE7+, FF3+, safari, and chrome. I'm not familiar with the different versions of safari and chrome but I haven't noticed much of a difference with them.
The rounder corners isn't really a big issue since it is only for the visual ascetics and in IE7, 6 (I believe), and 8 it doesn't break functionality. You can put a note on your site saying something like we recommend these browsers for an optimal performance.... Then you'd just need to check the browser they are using which you could do from the user agent either client (JS) or server side (PHP).
I wouldn't right additional code for other browsers if the functionality is still there.
Corrections to my coding/thoughts welcome.
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