To add to edboys post you can assess wear by measuring for ovality of the journals, I think there will be mixed opinions on the amount but, for me, more than 002 inches ovality indicates the need for a re grind, and a good micrometer will easily measure 002, as the bottom end is apart you might as well replace big end shells on the basis it should reduce any clearance due to wear that is present, and they do not cost too much, this is assuming enough miles have passed since the bearings were last done to cause at least some wear.
A machine shop will have the tools to measure crank journals and big end I.d. to much better than 001 accuracy but if you replace the shells there is no need to be doing that sort of measurement, again just my opinion!
A new main bush if fitted should be correctly honed to size, there is plenty of info on that topic on the forum......