Growing E. coli at room temperature?
Since b/w screening is a means to  selection, you want maximum selection pressure. This will be obtained by incorporating all optimal conditions. The many enzymes of the lac operon are going to function best at their optimal temp.

Why would you do this? If it is because you can't go in the next day it's better to do it when you're available. You're only going to doubt your results when you get them.