Aspirators vary, some are better for volume some for ultimate pressure. Water pressure in our labs, and hence ultimate vacuum, used to vary according to how much was being used at any one time. We didn't have non return valves and I'd have liked the safety net, do they reduce ultimate pressure?
Something I didn't get to try with a rotavap, but works well with regular vacuum distillation, pump down, seal off and cool the receiving flask.