Firedave, you cure LR by putting it in its own tank/container with some powerheads, a heater, and maybe a protein skimmer. I have mine curing in a 20 gallon bucket right now. Basically, you let the tub cycle, once it finishes cycling you can add it to your main tank. This may take several weeks to a month.
Phil, it obviously depends on the size of the rock and the size of your tank, but yes, it is better to add LR a few pieces at a time in an established tank. You can still buy it all at once though and put it in a tub to cure (as described to Firedave). If you don't have a tub, heater, powerhead, it would probably still cost less to go buy them and get bulk LR than it would just to buy the LR piece by piece at a LFS. As to assuming the LR is safe because it has been sitting. No. You can't assume that. You can assume it will take less time to cure though. To test if it is cured or not, I'd put it in a bucket and fill it with tank water. Let it sit for a while and then do a water test. If there is ammo in the bucket, you know it isn't cured LR yet.