Unless you have a planted tank, you should service the filter weekly. Why? The filter is trapping detritus (fish waste, uneaten food, crud). But that's not the end of it. It decomposes there and the water flow pushes the organics in solution (think of manure tea) through the filter and back into the tank. Oh the filter won't plug and could run this way for weeks or months....but the pollution is being pushed round and round. Of course, the weekly water change will remove a portion of this....but if the filter is serviced more often, there's less decomposed manure tea in the water so the water is cleaner and the water change even more effective.
Now in a planted tank, these organics in solution (the manure tea) is liquid fertilizer, much of which the plants will absorb, reducing amounts of required ferts.
Since cycling to the point where 5ppm of ammonia and the resulting nitrite can be dealt with in under 12 hours, I have never cleaned the filter sponges because that's where the bacs live. I have seen way too many "mini cycles" induced by someone "cleaning."
Been in the hobby for close to 4 years and everything I have read suggests you don't need to clean your filter. My bacs do such a good job that I don't even like moving the filter let alone opening it.