Take a look at two articles written by Mike Davis over at BIDN.com. The articles contain steps for SSRS 2008, but I imagine they might work for 2005 as well as 2012.

http://www.bidn.com/blogs/mikedavis/ssis/172/top-n-bottom-n-grouping-in-ssrs-2008

Here's his follow-up article that also deals with handling "ties" (which he calls "duplicates") for Top N:

http://www.bidn.com/blogs/MikeDavis/ssis/1875/ssrs-top-n-and-bottom-n-reporting-with-duplicates

--Pete