If you're putting your house on the market, you want the exterior to look beautiful. It's not the time to be frugal with water, because you don't want prospective buyers to see a less than well maintained lawn. Water your lawn a lot so it will look lush and green.
You want homebuyers that are looking at your house to get a nice feeling when looking at your home. Make sure that it's really comfortable. If your house is too warm and they're feeling sweaty, they might rush through. You want to extend the showing by making the temperature just perfect and pleasantly cool. Leave some bottled water for them, they'll appreciate it and think kindly of you.
Be flexible with showing times. People might look later since the sun is out later. Have your house always ready so that it can be available at a moment's notice.