When making squash, does it matter whether the water or concentrate are added first?

I'm not at all sure. It might be something to do with the way the two are mixing, but it seems more likely that this is some sort of strange psychological thing. If you mix them unevenly, some bits will be very strong, which your mouth will notice, and other bits will just be water which you'll ignore, so you'll only notice the really strong bits. I suppose it will also make a difference depending on whether you fill it slowly or with a gushing tap. At the end of the day, what is flavour? Well it's a molecule that creates a certain taste response. It shouldn't matter what order they're put into the glass. As long as they're mixed completely then you should get the same flavour.