I have no problem with the ball park sentiment. +/- 10 ppm on any ion isn't going to be a big deal. But if the water is going to change much more than that, especially hardness or alkalinity, then having a decent idea of the change will be helpful.
If you were trying to replicate a previous brew, it would be tough to do with a variable water. If you're just making beer, then the heck with it. RDWHAHB