Dry rock can leach phosphates but it depends on the source or if man made. For example dry Pukani, which is difficult if not impossible to get now, has a history of leaching phosphates. I have 150 lbs of it and guess what, it leached phosphates. I removed from the boxes, washed, then pressure washed, then spread out into 4 or 5 17 gallon bins, added tap water, bleach, power head, run 24 hours, emptied, repeated tap water and bleach, 24 more hours, repeat for another 24 hours, then emptied and refilled with tap water. Instead of bleach this time I added lanthium chloride to help remove the phosphates. Similar 24 hours, drain, refill, add more LC, 24 hours, etc. Once this was done I washed, then pressure washed, then aquascape, then into my tank to start the cycle...
I have phosphates in my tank but it isn't bad. In fact, it is manageable. I would say do your research and see the rock you are going to purchase and understand the source and read a bit more to see what some others who used it before you did or if they had measurable phosphates. I really think it just depends again on the source. Has far as to cure in or out of the tank. Again that is up to you. Once I was done with bleach, LC, and pressure washing I threw it in the tank and started the cycle. Worked for me but I can't say that will work for others.