I do upgrades on some machines, but for some servers (firewall, dns, print server), it just takes a lot less time to do a fresh install. I keep the configs, keys, etc. under source control and can put it all back faster than doing the upgrade.

It is also pretty good practice for anytime those servers go bad. It helps to be able to put temporary replacements in service from whatever I have lying around. I can save the hot spares for machines that have user data on them (e-mail, file servers).

I seem to recall someone (Theo, I believe) say that in order for an OpenBSD to maintain library compatibility with existing applications, you had to do an "Upgrade in Place" - and not do a fresh install.

I.E. The default approach, incremental upgrade, is the only way to ensure your OpenBSD system doesn't fail.