You'd want to take a sample of your preboil wort (well mixed) in order to figure your mash/lauter efficiency anyway. Yes, if you're using a hydrometer, cooling the sample in the freezer or icebath would be a good idea to reduce temp conversion errors. That's when you'll know if your gravity has fallen short. BUT!! don't forget to calculate what your boiled off gravity WILL be first. I've heard of people getting a low gravity with 7 gallons of preboil and thinking, CRAP! I missed the target.