You won't need to give them the block as all parts are balanced without it. If you have a 2 bolt mains block, you'd be surprised at how strong they can be, so long as you don't go stupid with the revs. When you put an engine together that may have had any new/different parts fitted, such as rods, pistons, crank, harmonic balancer, flywheel/clutch assembly or flexplate, it only needs for each of those parts to be slightly out of balance & you could end up with a large descrepancy in the total balance of the engine. A balanced engine is more likely to live longer than one that's been put together with unbalanced parts. Don't just think that because parts might be new that they're balanced from the factory, it's not strictly true in all cases.
Obviously it depends on what you intend to use the car for, if it's just cruising around with a very mild engine set up, then it's not as critical to have it balanced. But, as soon as you want to get some "beans" outta the thing, then it does become more important to have evrything balanced correctly, which in turn should give the engine the best chance of staying alive & in one piece. Just balancing the crank alone is not gonna fix anything I'm afraid, you'd just be chucking money away.
As an aside, the guy that balanced my parts told us of a story of a guy, if I remember correctly, that had an imbalance in his engine & the harmonics from that used to cause the wing mirrors to unscrew themselves.
hth
cheers...Nige