In my opinion, you WANT at least some disturbance to the surface of the sand. Be it via flow, or sand sifting livestock. A stagnant sand bed will start fouling a tank quickly. I believe this is why bare bottom tanks have been gaining popularity in this hobby for many years.
It is a bit confusing if you think about it. While sand is not a good media for promoting the Nitrogen cycle, it can very easily disrupt it!
Detritus, in the form of uneaten food and fish waste will collect on the surface. This is fact. If you do not have a means of disrupting this decaying material, it will stagnate and cause issues. The most common remedy for this is to increase flow. The thinking behind this conclusion is that moving water will displace the particles that settle on the sand bed and thus, prevent stagnation. The theory is sound as is. Unfortunately, the reality is that the results often are that the flow rate is excessive and ultimately detrimental to the system as a whole.
I prefer to utilize sand disrupting livestock as my primary means of prevention of this situation.
All of that said... every tank is as individual as its owner. What works for me, may not work for you. The variables that come into play here are incalculable. Your rock structure, your flow pattern, your livestock, ect ect.... This is an issue you will have to tinker with until you find the "sweet spot".
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