I'm not a hair dresser, but my hair dresser friend has taught me how to color (highlight/ lowlights) hair. I've been doing it for 8 years. I do weaves with foils just like they do at the salon.
I haven't been textbook taught, but what I do know is that every hair color has either a blue, green or violet base to it (thus imagine a bunch of senior citizen ladies sitting together with all shades of pink, blue or green hair).
By the way, it is rare that you get an actual hair dresser that works at Sally's (from my experience). She was probably repeating something that she had heard.
The purpose of ash is to take the redness out. If I use ash on a blonde, the hair will not process with a "gold" or "honey" tones, it is much whiter and and more gray. You have to be careful with ash because it can make you look more gray if you use too high of a level, but it works great for others. If you like what you are doing, keep doing it.
I've never done a "box" color, so I'm not sure how all of the levels process there.
Regardless, what color and level you use, it is pretty much an educational guess because you never know how a persons hair is going to process exactly. For example, I am a brunette (my base color is a level 5 if that means anything to you). I use an 8 ash in my highlights. But it doesn't make me look gray, in fact it processes with a honey color, but leaves me with no red.
OK, was that confusing enough? Good luck. I would take the advice of going to the hair school to get it done if you are worried. Nothing worse than a bad hair cut and color (at least for me)!
A.