If you're going to mix in an untreated room then you have to use references. Even if you know your room, you're still gonna want something to compare your mix to just in case. Eventually we should all strive to treat our studio spaces but obviously that's not cheap or easy to do. In the meantime it's best to utilize what you have and keep improving your listening environment progressively. This is especially important for beginners that are worried about mixing in an untreated space. Don't wait until you have the perfect room, get started now and make improvements as you go. Like I said at the beginning, use references and you'll get by.

Skrillex mixed a lot of his early work on Sony MDR 7506s and then *verified* on monitors before release. Same with Tame Impala's album Currents and Porter Robinson with his Worlds album (Sea of Voices is one of the best mixes I've ever heard as far as electronic music goes) I think what matters most is just references tracks.

It should also be noted that there's a range of untreated rooms, same as there's a range of treated rooms. The best room I've been in, a Northward room, just let you hear the music, no mental calculations to correct things (sadly I was just a visitor and don't get to work in that room all the time). I've been in some studios where they were treated but in a way that lead to poor translation. Some untreated domestic rooms are actually quite good, some not as much. Like Kevin says above, you got to have some references. Know those references inside out. How does the bass sound, is it as tight in this room as usual or is it ringing at 160? How do the mids sound, are they blurry in this room and you're not able to hear that tiny bit of slap delay panned 5% right at :36? Having a playlist of songs like this is the best way to cut to the chase and know what you're dealing with. These days there's also great solutions like VSX that give a level of performance I would've killed for 25 years ago.

When you do treat your room a bit of research will help to put your resources to the best use. For domestic treatment Gerhard Westphalen has great info as well as SDP*LA.

It is about knowing your listening environment: monitors/room and/or headphones. There are big name folks who mix in untreated rooms or with headphones, and some with questionable monitors or consumer grade headphones

Depends. A CONSISTENTLY bad room is useable if you learn it. A room that is bad in different ways every day (e.g. a small room that sounds different if the bed is made or not) is never gonna work.

I always like to think of visual analogies for audio stuff - it helps me answer questions like this. I imagine an untreated room as being similar to a screen that had a variety of issues:

color distortions (frequency response issues)
burn in (room resonances)
backlight issues (reflection-based phase issues)
Maybe others too.

Well, if I was a video editor working on a screen with all those problems, it seems like there are certain ways that I'd be able to adapt and make pretty accurate guesses about what a proper screen would show, but other things I would be way more likely to get wrong, or just not be able to perceive at all because they're literally absent on my busted screen.

Going back to an untreated room, it seems obvious to me that there are some ways a skilled mixer could adapt, but it would make their work a lot harder and involve a lot more guessing, and they'd almost definitely not even be able to perceive some issues. Checking on headphones would become a much more important part of the process. and that would involve its own things to interpret and adapt to.

So I think clearly room treatment is really important. But to that end, as others have said, it's not all or nothing. Incremental change can really help, even just by making it clearer how the room is affecting your sound. Anything helps. I've been following Jesco at acousticsinsider.com and his stuff helped me understand this topic so much better than before!

My take is that we hear our mixes differently when we come in from a long break or a few days - so in a room with build ups masking frequencies it's going to be much harder - headphones I think this works for for sure because there is not necessarily the build up and you can flatten with software