I think it is really a matter of personal preference. People (myself included) have been moving away from having a deep sand bed because they can become rather dirty overtime and many folks (again, myself included) are quite certain it is old dirty sandbeds that were responsible for rising phosphates in older tanks and resulting hair algae disaster.
I still have a sand bed in one of my tanks, because I have some creatures that prefer to burrow in the sand, like my yellow wrasse. In his case only an inch or two would be fine.
If you are going to be doing a type of reef tank where you you need high flow and very low nutrient levels, such as an sps only tank, I would say no sand. But if you enjoy burrowing wrasses, and other critters who need sand to go about their daily activities, then a moderate layer is fine.
Sand beds were mega popular for a while, and you will come across a lot of nonsense about how they are the easiest way to get 0 Nitrates, that you could feed heavy and the bed would take care of all the waste. That is nuts. People had 0 nitrate tanks before sand beds, and they still have them without. One key to a healthy reef tank is keeping your tank clean, with skimming, waterchanges, and removing of detritus! I sandbeds were so popular because they do work very well for a while, but all that fish food, fish poop, and other junk that goes into the bed doesn't fall into a black hole some where, you will have to deal with it eventually. . .
So add sand if you like the look, or if your critters need it, but don't think you must have it to run a reef tank.
