In cases where the engine is being rebuilt, I would measure the piston to deck on the four corners, and make note of the figures.
Then, after deciding on what piston/rod was going back in, I would ask for the deck to be machined- using the original figures taken at disassembly- as long as they fell within spec.
Now, this isn't for a high dollar race application. It is for the other 95% of the guys who rebuild their own engines and do not need to have the block blueprinted- which if done correctly (blueprinting, I mean) can be a very costly affair, depending on what is needed.
If I was working w/a bare block w/an unknown deck, I would have no problem doing a mock up after boring, before decking- as TI suggested.
This is not the "fast" way, nor is it particularly "efficient". But it assures that the decks will be where you want them to be, w/o incurring the costs associated w/blueprinting the block.