The decision to use or not use old bearings comes down to the condition of the bearings, the condition of the journals on the crank and the associated clearances.
If you're very familiar with looking at bearings, using micrometers and such - then you can check things out and make some decisions. Make sure you keep all bearings in order - so you know exactly where they came from.
What I've found that in many cases the bearings had some 'crap' go through them - maybe even during the disassembly process (rust, etc) - and they get score marks in them as a result (especially the mains). I always replace them - but that is just me and the bearings in my case are a small part of my overall expense.
If you're trying to be conservative on price, no reason you can't check them all out (along with the crank/rods) and see if they can be reused.
Take some good quality close-up photos of the bearings and journals - I'm sure there will be many opinions on here to help you make your decisions!
Good luck,
B&S