Can I leave uncooked rice sitting in water for an hour or longer before cooking?
That's fine. A lot of people actually do that on purpose, it's referred to as "soaking". It will shave a bit off the cooking time and won't negatively affect the rice at all. Give it a stir before you start cooking. You can drain and rinse it too if you want, that will give you very separate, distinct grains. Use about 1/4 less water for cooking if you do it that way, because the rice will have soaked up some water.