The reason it’s best to record as high as possible (without clipping) is because of the concept of a noise floor. But the importance is often over-stated.
First, I need to explain about dynamic range. The range of human hearing is ~120 dB for people with excellent hearing. Anything lower than that can’t be distinguished by a human. In a digital signal, every bit you use to record something you get an extra 6 dB of dynamic range. A 16-bit signal has 96 dB of range, a 24 bit signal has 144. I think the 32-bit float allows for 192 dB of range, but I can’t remember if the different format throws that off.
But all of that is just the digital domain, what happens when we enter the analog world (amplifiers, speakers, and recording)? Well, there are no perfect amplifiers or recorders. They all have a noise floor that they’re going to put out, and any signal that is smaller than that noise floor will be lost completely. Generally, this noise floor is about -90dB or so for good quality equipment, for studio equipment this is often about -105 dB. So most off-the-shelf amplifiers are unable to perfectly reproduce a 16-bit signal, let alone a 24-bit signal. This is why 16-bits was considered good enough for the CD standard. Even now, few amps will benefit from playing a 24-bit signal. All this is assuming a very loud signal (the sound should peak at the threshold of pain, +130dB), with perfect speakers, in a perfectly quiet room (which can’t actually exist unless you don’t mind suffocating). If we assume less than perfect conditions, then 16-bits is good enough for anyone.
So, exporting to 16-bits is good enough. Audiophiles can hem and haw all they want, but they still can’t pick out the 24-bit signal reliably in a proper A/B test.
However, things are a little different for recording. Any time you record something, you’re really recording two signals, the signal you want and the noise floor of either the analog input circuit or the digital format (whichever is higher). If you were to record “silence” (nothing plugged in), then you’ll really just be recording the noise floor (this is actually how you measure the level of the noise floor for you equipment).
Now, when you amplify a signal, you’re amplifying everything in it, including the noise floor. This is why it’s technically best to record as loud as you can, to keep the noise floor from coming up into the audible territory. For every dB you are below full volume, the noise floor will be raised that much if you amplify the signal all the way (I’ll explain why this isn’t a big deal later).
If you have good equipment, the noise floor of an input circuit might be way down in the -105dB range. As long as you’re recording at 24-bits or 32-bits, that’s where the noise floor will be (if you record at 16-bit, then the noise floor is stuck at -96 dB no matter what, that’s why I record to 24-bits). This is why you should use good quality equipment, it gives you a low enough noise floor that you have some headroom when recording. Note that none of this applies if you have an audibly noisy input, in that case you need to record as loud as you can.
Now lets take an “average” multi-track project recorded with good equipment. I’ll assume 8 tracks, each recorded to peak at -10dB, with no gain change. I’ll also assume the noise floor of the sound card is -100dB (this is what my Delta 1010LT is rated at).
Where will the noise floor of the final track be? It won’t be much higher than -90dB (if it even reaches there). Adding noise signals is tricky when working in dB, since they’re random (so someone correct me if my math is wrong).
A noise floor of -90dB is not perfect, but it’s damn good, probably . There is one problem with that example though. None of those tracks had no gain adjustments. In a real-world example, some of those tracks would have been turned down a bit, thus lowering the noise floor even further. This example can be played on most equipment without the noise floor being audible (even at a very high volume), even though we recorded each track at -10dB.
So that’s what I mean, if you have excellent equipment, don’t worry too much about recording as loud as you can. Chances are it won’t be audible in the end anyway.