There are two types of Iodine suppliments that I have seen. One is simply iodine, but the other is a delayed release or something of the sort. Basically, the second one is something you don't dose as often. According to what I hear, it's better to get a test and the regular iodine suppliment so that you can have better control over your iodine levels.
Also, depending on what is in your tank in terms of livestock and how often you do water changes, you may not need to dose iodine. Like you were asking, many people just use water changes to keep their iodine levels up.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars