I don’t think panning is an essential mixing tool.
This is because – in my opinion – it doesn’t cause any problems, and it doesn’t solve any problems.
I don’t think ‘bad panning’ is the cause of any mix problems. Try to think of the craziest approach to panning imaginable – say drums hard panned one side, lead vocals hard panned another side. Bass off-centre, etc… The opposite of common practice. Now listen to Sergeant Pepper’s Lonely Hearts Club Band. The mixes are great! The bizarre panning doesn’t let the music down. I don’t think unconventional panning makes for a bad mix.
In addition, I don’t know of any production problem (including composition and engineering) that is best solved with panning. Sometimes I see panning proposed as a solution to instruments clashing or something like that, but that sort of problem is actually better solved through tonal adjustment (such as EQ). The existence of that kind of problem is usually caused earlier in the process – a lack of decision-making around what elements of the mix are most important, and a lack of courage to thin out and push back less important elements. A good stereo mix makes for a good mono mix. If a mix doesn’t work in mono, chances are it’ll be pretty dismal in stereo too.
Panning is also something that’s commonly understood. I don’t see people asking questions about ‘how to use panning’. Anyone with headphones or two speakers instantly understands how panning can be used to best support a song. The only time I’ve heard panning used in a way that didn’t support the song was by an artist of mine who had made some demos using only her Mac Mini’s built in mono speaker. And as you can imagine, the bizarre panning was the least of my worries. In all the many amateur mixes I’ve heard (from people who actually care about mixing), panning has never been a problem.
The only way panning can be a problem is if people rely on it to solve problems that are better solved using other tools or techniques. In which case panning is not actually the cause of the problem – and neither is it the solution.
So feel free to pan however you like – you won’t do anything wrong. You’ll know if what you’re doing doesn’t support the song. And if you think it’s ok, it most certainly is.