I realize it depends on the space, but generally speaking, what microphone technique do you use and what type of mic? XY, ORTF, spaced pair?
Honestly, I just use whatever I have - even if all I have is mono. Never let the equipment get in the way of a good roomtone (or for that matter, ambience). I certainly agree with @Guido that high quality gear is best suited for roomtone to obtain the most robust S/N, but beyond that, I'm pretty loose about this type of recording.
Roomtone is one of those things which is quite forgivable in post for creating "pseudo-stereo" by slipping the channels out of sync a ways. I actually find myself doing this a lot, especially since I am often layering a few textures to get the final gestalt I'm seeking, so this is a case where I use the technique to alter the spread of each texture (such as, maybe, a roomtone edit for a warehouse or rotunda).
And do not under-estimate the necessity of mono textures in a BG edit - Every show I've worked on has a set of dedicated mono BG tracks. One reason is that in the stage's routing, it provides them a decided discrete Center which can be mixed against DX to help those problems (which is why we always try to give at least one "airy" roomtone element even in a place like interrogation rooms or tonal spaces at low level), and it can help mesh the LCR imagery (such as a party crowd edit - a mono walla in C, then a true stereo walla in L/R and maybe Ls/Rs, and then maybe some intentionally "pusedo-stereo'd" group ADR-style wallas in L/R and Ls/Rs to create immersion and a sense of 3d space which pops).
Right now, my go-to rig is MS. I haven't recording much roomtone type textures with it yet, although I am intrigued by MS quite enough now that I'm curious how effective it is. But I've recorded roomtone (or roomtone-like spaces) in mono on my iPhone, stereo with a H4n, and with an NT4 X/Y on my SD 702. All have been used in all sorts of shows, no holds barred. If it matches my needs at any given time, I'll use it, regardless of what I recorded it from.
So in the end, to directly provide my own answer to your roomtone question: I use both whatever I have on me and gets the job done.
Take omnidirectional microphones! In contrast do cardioids, omnidirectional microphones captures much more low end.
This gives you the andvantage that your room tones will also have something in the lows instead of cardiodid records. Cardioid microphones cut's bass frequencies which is good for voice and music. For rooms, it makes it thin.
In post production, room tones usually get played on a very low level, so the omnidirectional record will give you more 'character' also on low-level playback instead of a cardioid record (ORTF, XY...), which only turns to a simple hiss. If you record with MS, the side-microphone is also very similar to an omnidirectional and records lot's of low end. Also, I think it must be clear to use only fine equipment, otherwise you have only hiss from the preamps.
If you don't have time to hang around long hours in basement and dark floors, have a look at my room tones library: https://www.soundeffects.ch/en/sound-libraries/soundeffects.ch-products/roomtones-v01-sound-library.php It has more than 130 room tones atmospheres from stereo to surround. Also included in the library are a big set of impulse responses to create rooms from stereo up to auro 3d 11.1. https://www.soundeffects.ch/en/sound-libraries/soundeffects.ch-products/ir1-impulse-response-set.php
Soundfield SPS200 A-Format (4 channel) which I can decode into 5.1, 6.1, 7.1 or 8 channels.
I have recorded room tones in room corners. The sound is louder and you get better signal to noise performance. David Lynch is using lots room tone sounds. I read that he is recording movement of air through closed door.
David Toop says the emptier the space of sound, the greater the apparent volume of sounds within it; the lower the level of auditory background, the more intense the listener’s awareness of minimal interferences.
I have also manage to do room tone sounds from field recording example using distantant city sound and playing that thru lowpass filter and wet impulse reverb. That gives lively room tones which are easier to listen for longer periods. Also change filter settings and try different impulses example. room impulses gives you lots of different results using just one source recording.
In My world, Room Tone, is not what you folks are describing above in general.
Room tone is what is used in a dialog edit to replace or "hide" imperfections or other issues with the general room tone of the recorded dialog. Room tone doesn't need to be recorded in stereo. Room tone are often better the "worse" they are. Room tone CAN be used as the basis of the sound of a specific place, but in general it is not. Room tone in general is not part of the sound design but part of the dialog edit.
But a room tone can be used throughout a scene (especially a quiet scene) to generate a stable center sound if very little ambiance is needed.
Yes you can record the tone of a room In stereo with super awesome mics, but those types of recordings rarely fit with dialog recorded with a shotgun or a lav. And that is what is needed.
I suggest avoiding the use of room tone for what is essentially a ambiance or background recording of a empty room. The intended use is simply different, and we have enough words with a split personality in the sound world as it is. Let's not create another. Or perhaps it's to late already...