I generally find MySQL Sever sufficiently tested, meaning at least minor version upgrades rarely cause the problems. Of course it is not perfect and I remember number of big issues when some releases could not be used due to behavior changes in them and when something had to be rolled back in the next release.
But generally I think MySQL is doing much better than a lot of other projects in this respect.
Major version upgrades become more painful recently. In MySQL
3.22->3.23 upgrades or 3.23->4.0 upgrades you really could
just swap binary and try out new version and if you need to get
back you just swap binaries to the old version. I think it was
great.
In MySQL 4.1 and later more caution is required because you could
screw your data, for example by specifying utf8 as default
character set together not to mention infamous timestamp format
change which forced to change a lot of applications.
MySQL 5.0 added more …