Breaking bushings in is an interesting one. Some seem to need it, some don't.
I had some Aces not long ago. Had the nut all the way off, flush with top of kingpin. Initially they felt really fluid. Then they started feeling harder, which was really annoying. However, a few hours more and they mellowed out again - feeling only slightly harder than how they'd originally been.
It's worth having a look at the opinion about trucks on Skateboarding Is My Lifetime Sport blog, because he often mentions if he feels OEM bushings need breaking in - or if an alternative is better:
http://skateboardingismylifetimesport.blogspot.co.ukSome of it is a bit out of date, but it's a good basis.