Not everyone out there is a fan of tuners. This fall I have been working on a specific warmup routine to improve intonation, and I do recommend the use of tuners as tools for improving intonation.
But before going there, it is worth mentioning of course that playing against drones and tracks of any kind are really helpful. I enjoy going back to The Brass Gym for horn for this reason (more here on this publication).
Objectively though, you know having a physical device that tells you if you are in tune is also a very helpful thing. One suggestion that I have found useful is to have more than one tuner handy. This may sound extreme, but at home I have three I regularly use, reduced down to two when teaching.
The one I like the best for general use is the least sensitive one, my “old standard” Korg CA-1. What is good about this is it gives the “green light” over a wider range of pitch, you can be slightly high or low by several cents and still get the light. Why this is good is that in reality intonation in a group is more about being real close to begin with and making small adjustments by ear to make it better (this is why drones are so useful for working on pitch as well).
The next one I use a lot is the Tonal Energy tuner app, on my phone. It has a happy face that only becomes happy over a slightly more narrow range. There is much to be gained by aiming hard at that smaller target, especially during practice. But it still has a bit of a target, and you get the partial happy face when you are close. (For more on this tuner and even more notes on working on intonation see this article from a few months ago).
The final tuner I use is an older Seiko ST757 that has no face or green light, it just gives you exactly how many cents you are off from perfect. And you will be off from perfect virtually all the time, it is incredibly difficult to play every note exactly in tune with equal temperament. But there is a time to really beat yourself up a little, you need to be clear about tendencies and work them out.
In relation to that last point, there is one final thing to mention for today. Intonation with yourself is a function of two things. One part is you and how you blow your horn; the other part is how your horn is set up in terms of slide positions and overall design. If you blow it very differently than the way it was designed to be blown it won’t play exactly in tune. Presumably the horn, if made correctly, can be played in tune; presumably the people the builders worked with could play it in tune as constructed with the slides in some reasonable positions. But if you really can’t figure out how to get it set up and to blow in tune, maybe the problem isn’t you — it may simply be time to get a different horn — and tuners will make that decision clearer to you.
To conclude: besides the use of pitch comparison with drones or tracks in your practice, it is also well worth owning several tuners as tools toward the goal of better intonation and better horn playing.