I don't squint as much as I should. Actually, what I do is kind of weird (and I don't endorse it), I slightly cross my eyes while closing one eye. This gets rid of all the details, and I can focus more on color and value. I don't know where I picked this habit up, but I do it constantly without even realizing it anymore. But for some reason, squinting, or doing the crazy cross eyed thing, helps with almost every aspect of painting. It seems so simple but it really seems to push everything else out and allow us to focus. It is an invaluable tool.