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Not THAT essential IMO as long as you replace filters every 6-12 months. If your water is very high in TDS I would get a meter. I personally don't have one and replace every 6 months because I make a ton of water.
I test my water every week or so to make sure rodi is doing ok. My water isn't too bad where I live 150 ppm. But still knock that down to 0. With ro/di. If u have a filter I think the TDs meter is essential to put out pure water but that's just my opinion.