I don’t know, maybe maybe not, the answer will vary depending on the person. The point is, it’s useful tool to help you figure out if your in the 5% or not. If you don’t measure it how will you know?
It’s not a fixed amount though, it’s a percentage for a reason. 20g is an average, and many people don’t fit the average - 5% means 5% of your calories come from carbs. So you need to know how many calories you’re eating, and then convert that to grams to know.
E.g. For me, if I eat 25g carbs a day I will only be in light ketosis and not efficiently carb adapt. I’m small and my resting metabolic rate is 1000kcals a day. 5% carbs for me, if I eat the full 1000kcal, is a minuscule 12.5g- half the average. 2 cups of tea with milk and I’ve already used half of them. 1 avocado has 17g of carbs. Hopefully after I’m adapted again I can up my calorie intake and maybe go to 10%, but I’ll probably go carnivore.
I could easily have been one of those people posting that my keto diet was failing and I wasn’t carb adapting and putting on weight even. The maths explained to me why and I resolved it myself, so I think the usefulness of understanding the numbers can’t be underestimated.
I don’t need to buy a glucose monitor, I can feel it and that’s free 