First, there is NO "stable version" of any product (Not just with Symantec, but with everyone) .... The product's functionality depends on the environment... mainly, one Mr. Windows.... But its always recommended to use the latest version as the developers claim that it addresses the known issues with previous versions....
AFAIK, RU5 is working absolutely fine... :) ... Haven't got any headache so far.... But due to the advancement in Auto Protect architecture, it messess with some in house applictions.... Like Quickbooks... But otherwise, its fine.... If you run into this application issues, go for MR4 MP2... the next best..
So, my suggestion is RU5..
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007121216360648