Sometimes, when I rebuild my Sun machines, I update the OBP (OpenBoot PROM).
Not always, and not necessarily to a systematic pattern. Part of the reason for this is that I don't always have a convenient window to do so, but also I'm not entirely sure whether it's really a good idea - it's not broken, so why fix it?
I have had SunService complain about non-current OBP versions, but without any particularly good justification. And I have had one case where an OBP update actually stopped a system working.
It's easier to update systems now - US-III machines (most of them, anyway) can be updated from Solaris with a script, which is good. I just have to put the keyswitch into normal (which involves physically going to the machine) on servers. Older machines are slightly more tricky.
I know there are some cases where I have to upgrade OBP. For example, before a CPU upgrade so it will recognize the new processors.
I wonder what other people do as a strategy. Do you religiously upgrade OBP, or avoid doing so like the plague?