Depends on what you want the cells to accomplish. At 37C (stem) cells will proliferate in the absence of differentiation, if differentiation factors are absent. Conversely, (stem) cells growing at temperatures above and below 37C will differentiate without differentiation factors being present. For example, endodermal stem cells will differentiate into pancreatic progenitor cells if grown at 35C, without any pancreatic progenitor cell inductive factors being present.