Almost never. None of my numbers say a water change would benefit anything.
A lot of people need to step back and find the root reasons they are doing a water change. Does your tank actually need one? What are you accomplishing with a water change? You certainly aren't replenishing elements unless they are at zero and your salt happens to contain that trace elements. If that's the case, you should be dosing it. Not relying on a water change to add it back.
Remember, those who encourage water changes the most are salt companies and the companies selling salt
I use natural seawater I collect myself. I'm not spending money on salt, and I can absolutely see immediate benefits from my water changes.
- The removal of visible waste/detritus during the change. I could reposition powerheads, move around rock work, and add filtration to try to get my flow just right so that this stuff doesn't collect in certain spots. But I'm running a nano and don't have a ton of room to play with. The water changes are a much easier way to handle this.
- The micro flora and fauna in the water I collect triggers a pretty wild feeding response from all of my tank's inhabitants. The coral in particular fully extend during each change and are growing rapidly considering how young this tank is (by far the best of all of the reefs I've ever run). I realize that adding reef roids or something similar could have an equivalent effect, but that costs money and the NSW doesn't trigger nearly the same nutrient explosion that overfeeding does.
- 35-40% (5 gallon bucket) biweekly water changes are enough to have an impact on nutrient export in a 13.5 gallon tank.
- I enjoy going to the beach to grab my water. And even if the 3 points above didn't make a difference, it'd still be worth it for me.