I asked myself a similar question these days.
I guess the semi-official way is to follow the sticky-thread:
So far I never bothered to update. I think you only need to do it if the change log convinces you to.
But yes, I totally support the idea of a somewhat official documentation with news on updates.
There is the cleverly hidden, seemingly outdated wiki here: ⚡ PureOS Wiki
Would be nice to have it linked somewhere on puri.sm or on pureos.net, and updated more frequently, informing about the latest updates by the developers 