LMSC Yes, I measure for every brew (unless someone just pops round for a cup, then I use a known good method).
What would 19%EY be telling me, other than it is the typical range? For a single bag over 10 brews, +/- 0.4% would be normal variation. There are a couple of ways that people calculate EY adding another potential +/- 0.7%EY. You need more context than just the EY to make sense of it, in the same way that 18g of beans is 18g of beans however they are roasted, or taste.
If you are consistent, you are never going to see readings that are out of the normal range.
If I have done a minimum 10 brews that average a typical EY of 19-21% for example, with a good average liking score, then I make a brew that measures 19% I wouldn’t change anything if it tastes good. If I felt pushing on the extraction a % or so would improve it, I would then move to a method/grind setting that extracts a % more on average.
If you use a different brew method for the same bag of beans, your EY may change too.
It takes another 30-60s or so to take the reading, costs 2x AAA batteries every year or two. Why not do it? (Drop some 0TDS water in the refractometer, weigh & grind beans, start kettle boiling, Zero refractometer & clean lens, brew, stir brew, drop sample on refractometer, rinse brewer, measure EY, clean refractometer & drink)
On the other hand, if you stick to 1 brewer, grind setting & method you could just not measure after dialling in, then only take some more measurements if making some remedial changes, to see if you stay on track/improve?