I'm going to have to somewhat disagree with being mandatory. While it is ideal to keep your tank as clean as possible I don't feel substrate vacuuming is mandatory. Reason 1: I don't clean my substrate, but I use sand and not gravel. The fish in my tanks constantly stirs up all the waste matter on the sand and sends it floating throughout the water column where it ultimately gets picked up by the filtration system. Also, I do 50% wc every 3-4 days. Reason 2: Back in the days where under gravel filtration was a standard, people hardly, if ever, clean out all the crud under the ugf panels.
While I'm in no way advocating not to clean the substrate, because it is a good habit to do so, but my own personal experience is that it is not mandatory.
Hello; I am one who kept tanks back in the days. I ran UGF's and had not caught on to the notion of doing a gravel vac/cleaning for over a decade.
It was my common practice to break down a tank every year or two back then for a general cleaning. It was fortunate that my tanks were in the 10 and 20 gallon size. The substrate, either gravel or sand, would indeed be nasty. This was labor intensive but not complicated. A simple rinsing of the gravel was the main thing. I keept the live plants and fish in buckets or another tank.
I now use a gravel vac on my tanks. It does not prevent the substrate from becoming loaded with "gunk" but is does slow the buildup process down. I can now run a tank for many years before the need to break one down. I could possibly see a tank never having to be broken down if a person was dilligent enough.
I like live rooted plants in my tanks and am reluctant to be too aggressive when doing a gravel vac around them. This allows the "gunk" to build up in a heavy planted tank. The plants do, however, seem to help keep a tank run in better condition and i think they are a positive.
One practice I have used with success is to keep a cleaned amount of substrate on hand. During a typical water change some amount of the substrate is siphoned out with the water. That substrate is replaced with the cleaned and the dirty is cleaned for the next water change. This way over a series of water changes i can eventually clean most all of the substrate. Has the advantage of not being a big enough tank disturbance to cause issues. In theory a tank could be run continuously, but i have not quite managed it as yet.
Mandatory = NO. Like flossing teeth, it is a good idea.