I understand the need for buffers with large systems. A 200g tank would require about a 50g water change once weekly to keep up with the 25% rule</em>, even 10% would be more water than I turn in my display tank, and who has the time for that. Buffering is the way to go on large tanks as it extends the need for water changes.
If on the other hand you have a 25g cube, then a 3g to 5g water change weekly is actually easier than testing for what you need, dosing the tank, and then testing again to check the parameters. As when using additives, thats the way it should be done.
On my 30g, I typically just use two 3g buckets for water changes. One bucket for clean make up water, one for syphoning dirty water. Just the 10% keeps my tank happy, but stick with 20% to 25% (until you're acustom to your tanks needs) and let the water age overnight (preferably with a heater set at tank temperature) before adding it to your tank.
I've personally never had any issues with salt mixes not having the correct amount of buffering agents, but have heard of this happening, but its far from the norm. The Sea-Chem rep at our last meeting hit on that topic for a moment, but again, typically, salt will have the buffers you need.
But do always test your water, at least until you get a feel for your tank.