Given my personal familial situation, this was of great concern to me. Ultimately, I accepted an offer at the only school that made me an offer, but honestly, a major factor of consideration for me was the feasibility of weekends off. I spoke with some students at the school I will be attending in the fall and they said that, though it does rely largely on your project, the PIs in general, do not seem to frown upon not being in the lab on weekends. It was important to me to be able to be free on weekends, either to go home or for my family to come to me. The consensus was that I am at the mercy of my project, so choose wisely. This is probably less of a concern for non-science people.