To a certain extent it depends on where you are in the world. Twice in my flying career I was given clearances that had I followed them I would have bored holes in hills. Yes, when under radar control and being vectored for the approach, ATC have responsibility for terrain clearance. But I would still be very wary and check every cleared altitude and query any clearance that did not correspond to my own navigation (charts and/or ND). Only when you are postively estabilished on an instrument approach procedure should you descend below MSA/SSA. We all can make mistakes and it is good airmanship to check every clearance, however good ATC may be.