You set them and forget them. This has been discussed at great length.

You could set them at low volume if your ears and the amp and speakers are linear, but they're not, so you'll need to get it running properly somewhere.
You use them to tweak the distortion sound, it's an extra TMB stack after the distortion. It is a set and forget thing

I mean that you will not be able to use HRM ro properly tailor your sound for full gig volume when running it at home on the lowest audible setting, you will need to turn it up as loud as you intend to use it, with the chassis out of the case

I wondered about external trimpots when I ordered one, but if you read all the old threads you will find that people have no problems with this. People who have had the trimpots exposed tend to leave them as-is anyway.
I cannot imagine why the settings would need changing from one gig to the next to compensate for room size, and cannot believe anyone would tweak these mid-gig.
My point is simply, "crank it up to normal volume while you set the pots", do not preset the amp tone at conversation level volumes.
True to the original design, the trimmers for this _extra_ tonestack are inside the amp, right on the signal path and they set up the character of the distortion. the front panel knobs still work too when you're playing live.
If you're having a lot of trouble believing all this, you're going to have to track down one of these amps to try it out, or just buy one based on the good reviews by serious players you see everywhere.

There are other trimpots / mods that could be usefully tweaked for a single performance: PPIMV, half-power, VVR, OD trim
However, not every knob should be on full time display on teh control panel.  I would not want PI tail or bias on my control panel:
HRM is something that would be more convenient on the back panel (perhaps with screwdriver adjustment), but since no-one seems to change their settings, everyone lives with it, you can have it made accessible, but as I said, I doubt you would ever change the settings.

If you like HRM pots on the frontpanel you better go for a Two Rock amnivirsary or a certain type of Glaswerks amp. 
These brands did extensive research to take the HRM concept a step further without ending in a humming amp.

FWIW I like my HRM od channel only with the PAB on. So I tweaked it at gig levels as a extension of the clean channel sound wise. Now it's untouched for almost 3 months(set and forget).

Hi Claus,

I forgot, but if you're really interested I can open up my amp again, set and forget applies here...

Check the manual: http://www.ceriatone.com/Manuals/ceriatone%20hrm%2050w%20100w%20manual.pdf

Tonestack on the frontpanel is working for clean and drive channel. If you engage the PAB, the tonestack on the frontpanel isn't working.
The internal trimpots for the overdrive tonestack are always working for the drive channel.