I've seen 1" sandbeds with black patches.
Nassarius snails aren't usually good enough to clean the sandbed, but they do help. If your tank is 50 gallons or more you could consider a small conch. They need a lot of food though, so you need a fair sized sandbed to provide them with enough food. They are good in the sandbed because they eat bad things, and leave good things.
Personally, I think any sandbed requires quite a bit of maintenance to keep it in top shape, so I would agree and say that if your critters aren't doing the job 100% that you should help out with siphoning, and at least stirring the top 1/2" or so. If you do start stirring though, don't be surprised if you have a bit of an algae bloom for the first few weeks you start since you will be releasing a bunch of crap stuck in the sand.