Ammm, I think you just tell it to use wifi when available. I haven't set any locales or anything else and when I get home, it just connects to my router. Seamless and no action on my part.
The issue with this is, that it will use extra battery power.
Leaving the WiFi antenna on all the time, means that it will be constantly checking for service which in turn equals more battery drain.
For a while I was always turning WiFi on when I would get home, but 3G is quick enough for almost any thing. The only time IMO WiFi is ever really needed is when you're downloading a large file or video, other wise 3G is usually sufficient.