Tips on training bayes?

Hi,
Now that I'm using SA4, and my bayes database is quite old, I'd like to
retrain it with new ham and spam. I hoped someone had some pointers on some
of the gray area and what you consider to be spam and ham.

Are reliable newsletters, like those from, say, a trusted news source where
the user opts into the subscription, considered ham? What if you can't
quite tell whether it was opt-in but it doesn't hit any current negative
rules like blocklists or sender reputation or DCC? I'm assuming it's best
to leave it to later be reported as BAYES_50?

Are unsolicited emails always considered spam?

Thanks,
Alex