A problem with "toe" is (and I have struggled with this) that if this isn't specifically taught as a "low" concept, it can really mess up the sound in the high register. The oral cavity should not be the same for C5 as it is for C2. Mine was, and it was making me engage the throat to reduce the size of the air column, and that added way too much complexity. "Tee" for C5 is much simpler than "toe".I've been really thinking about this and working with the concept in my practice sessions lately. Wouldn't you know it? All along, I think my tongue has been screwing up my high range. I have written before about my issues with my high range, and Jon has too, and one little comment on Facebook has really made a big difference. I started raising the back of my tongue when I ascend in my range, and not only is my sound clearer and sweeter up there, but the notes come out much more easily. I was even squealing away on an F5 today like I haven't spent months trying to find it.
tl;dr - articulation is separate from vowel sound which should vary according to range
Training myself to put my tongue in the "ee" position will be the hard part, but at least I'm on the way.