I'm using Avast 5.1.889 Free, and SpyBot Search & Destroy with it's full immunizations. I have SDHelper enabled for Internet Explorer, but have TeaTimer disabled. I have no conflicts running this way.
The reason I disabled TeaTimer was that other family members use this machine, and might not answer it's queries properly causing some software to not be able to configure itself properly. Although the more recent TeaTimer asks for approval far less often by design, there's still the possibility.
I do think that the immunizations are of use to block some malware, and I also use SpywareBlaster's immunizations for the same reason.