Since 7-TET has 7 tones, I thought the tone are okay to be labeled CDEFGAB.
But actually, the "E" is closer to just ratio 6/5 than 5/4, and "B" is closer to 9/5 than 15/8.
So is it really okay?
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Since there are 7 pitches per octave, and you have 7 letter-names, I'd say it's OK to do what you're already doing, because anything else would be less convenient.
A temperament with 7 pitches per octave doesn't distinguish between major and minor intervals. For example, between major thirds and minor thirds -- every third is two steps and every step is 1/7 of an octave. So sharps and flats have no effect, so there's no point using them.
I think there are 2 main reasons to be talking about n-TET scales:
The first reason only involves n-TET scales where n > 12. So if you're playing around with 7-TET, I think your usage is most likely in the 2nd category. So, with that in mind...
If this is pure theoretical musing, or for something that only you will ever see or read, then call them whatever you want. For instance, if you're assigning 7-TET notes to a midi keyboard so you can play around with the scale, then using the white keys would make the most sense since physically they are equally spaced.
If you're writing this down and you think other musicians will be reading it, then whatever you do you need to explain it very clearly so others will not be confused. For this reason, I would avoid using the standard letter names ABCDEFG because it will invite confusion since people will assume you mean the "white key" notes.
If you use ABCDEFG, explain how your usage is different from the normal usage. If you use something else, explain that too, otherwise it might just seem nonsensical. Best practice is to always carefully explain anything non-traditional, whether a traditional element being used non-traditionally, or something completely novel.
Personally, I'd use numbers. 0-6 makes sense if you're going to use them for a 7-TET version of pitch class theory. 1-7 might be more useful in other contexts because of how they loosely correlate with the 7 scale degrees of the familiar diatonic scales.
*I'm not an expert on this subject, but I think it is generally assumed that only a subset of the notes will be used, those which most closely match the just intervals. Those notes are given the traditional note names while the unused notes go unnamed.