You can use tap water to fill a saltwater aquarium when you do your water changes, but it really depends on the area that you live in and how good that water is. Most water is safe to drink and bathe in, so therefore, it's got to be safe for your aquarium inhabitants, and it usually is. A lot of times, there's phosphorous, arsenic, and other algae-promoting nutrients in the water that will result in algae blooms. You're trying to benefit your aquarium by removing waste, yet you're adding other forms of waste or nutrients that are going to promote algae growth.
For those reasons alone, I would always recommend using filtered water, either reverse osmosis, a deionized filtered water, or bottled water as opposed to using tap water. If you're in a bind and you've ran out of water, and your tank's a little bit low, you can use tap water. I would use a dechlorinating agent, any of the dechlorinators will work. You want to remove the chlorines and the chloramines. It's also better to let the water sit overnight so it has a chance to degas.
Then also, keep in mind the temperature. If you're using tap water, chances are it's either going to be a little bit warm or a little bit cold. You want to make sure that the temperature is matched to your aquarium so you avoid swings, so you don't shock the animals in your aquarium.