Currently, when I code I use:
-webkit-border-radius: 4px;
-moz-border-radius: 4px;
border-radius: 4px;
But based on my tests, it didn't make any difference with any modern browser ( Chrome 33+, opera 25+, safari 8+). Internet Explorer 8 doesn't supports this property, but the vendor prefixes won't make any difference.
Is there any reason to keep them?
Use http://caniuse.com/#search=border-radius for checking such
Conclusion: No need for adding vendor prefixes for border-radius, as its supported in all major browsers (and IE9+). If you really need border-radius in IE8 check out: How to apply border radius in IE8 and below IE8 browsers?
But in 99% of cases border-radius is not crucial to a design. Employ the technique of graceful degradation and leave IE8 with square corners