An ATO is critical for a reef tanks stability....I learned that early on and should be stressed more to beginning reefers. Unless, of course, you are sitting there watching your tank all the time and every day, so you can do frequent top offs. But then I'd have to say..."Hey Man, go get a job. Reef tanks can be expensive and those corals aren't gonna buy themselves!"
But seriously, I was nervous leaving my tank too early on, and still am to tell the truth. I try to have a fellow reefer or dependable friend (with detailed instructions) check in every day or two, I have a spare return pump all set up to go with a union in case of failure, a spare powerhead, an auto feeder with pellets and flakes and an ATO reservoir that has enough volume to cover my time away. The more time and money that I have invested in corals forces me to prepare more for when I am out of town.