If your going to just put sand between the deck stones then you will not need the caulk. Two things to think about though, one don't let the deck stones touch the pool coping stones and two if you are doing a sand set on the deck stones and water is allowed to to drain down between the coping stone and the deck stone then make sure the water will drain away from the bond beam of the pool (this is important if you live in an area that has freezing weather). If you are going to mortar between the stones on the sand base then you will need to caulk between the coping stone and the deck because you do not want a void space and you do not want mortar between them. Hope this helps.