Time and time again I see people saying that we need to "open" our pores to clean them and "close "them up to prevent them from getting clogged. The truth is that pores don't have muscles to open and close. When skin therapist's use steam during facials it's really to relax the skin which makes it easier and less traumatic to take out black heads. Using cool water or a cool spritz at the end of a facial helps to temporarily tighten the skin, but alas will not "close" pores to prevent blackheads.
So what temperature water should you use to wash your face? Does it make a difference? Warm water is still a great choice since it will help to warm up and dissolve oils and waxes from makeup, therefore making it easier for your cleanser to do it's job. If you plan on trying to remove your own blackheads (which I would discourage if your a "picker"), putting a warm, damp cotton pad over the spot could help relax the pore reducing the chance of swelling.
As for a cold water rinse? While it won't close pores, it definitely feels refreshing, can help reduce swelling and redness, and can temporarily tighten the skin.