Having worked in construction for a while, but never built a residential pool, I'd say that it -might- be cost-effective to have a geotech firm drill a hole in the center of the deep end location (prior to signing contracts) in order to determine if there is any rock. I say -might- because that alone can cost a few hundred bucks. It's a risk aversion strategy that may not be justified at this level of total project cost. Without that information, you take on the risk of encountering rock or a missle silo. But, if you try to lock in the contractor to a fixed excavation cost, they -should- pad the dollars to pay for the risk they are taking on. So it's a trade-off.

In urban areas there is usually a well-known geological pattern that has been developed over time, and a good geotech firm will have that on file. A good pool company that has some experience will know where to get this information and shouldn't be too surprised with what they dig up in your back yard.