I usually make every update on the stable branch, no problems for years and years even passing from a release to another.
My servers live with a daily, manually run, upgrade-system (great tool) and they have been updated and upgraded from Woody to Lenny quite simply.
Of course I do not make the updates via cron, I get only the updates availability this way.
In case of core packages such as glibc I first test the update on a non production machine, of course.
I strongly recoment upgrade-system, as it runs deborphan too and helps to keep the system up to date and clean at the same time
My 2 cents
Note that inside a given release (Lenny for examples), could perfectly be a "sub-release", say 5.0-r1, 5.0-r2 and so on.
As reported in the wikipedia page on Debian, a stable release can be updated to fulfill security and/or usability issues:
"stable, presently aliased lenny, is the current release that has stable and well tested software. Stable is made by freezing testing for a few months where bugs are fixed in order to make the distribution as stable as possible; then the resulting system is released as stable. It is updated only if major security or usability fixes are incorporated."
The short answer to asdmin's opposition is that it is perfectly possible to have non security related updates even inside the same major stable release