Some people say we should never use an electronic tuner. I think tuners can be very useful if used correctly.
It's not a good idea to clip a tuner on your instrument and tune every note on sight while playing in a group. It's more important to tune to the group than to a machine so you'll have to learn to do it by ear . With a bit of practice, it's not that hard. Also be aware that notes are tuned differently depending on their function in the chord. ( major thirds are tuned lower, minor thirds higher etc.) So when you are working on brass chords, you can only use the tuner for the root of the chord. The tuner can be used if we disagree on some notes but not all the time.
When teaching a group of beginners or young students, it's easy to lose the pitch when the kids are going in all directions. A quick check on the machine will help putting the group back on track.
It's also very useful when you need to arrange your or a student's slides.
Intonation can be a very subjective thing. Depending on how our ears have been educated, we ear things differently. The way we "feel" the pitch is also different from one person to another. I played an exam once where one of the three judges said I was a bit flat, the other one said I was a bit sharp and the last one said I was just right! That happens a lot actually.
I personally prefer someone using an electronic tuner once in a while to someone who pretend he hears everything but doesn't really have a clue. There's nothing wrong in admitting you don't hear everything like a machine. As long as you can tune yourself by ear in the group, it's ok to take a couple of seconds to check a note here and there with the machine.
Don't be too precious to about your ear. At the end, it's not about who's right or who's wrong. It's about how do we play in tune together. If tuners can help us a little, than why not?
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.