There really isn’t a guideline for best performance here–it’s all about giving you options to deal with volume in whatever way makes most sense for your hardware, configuration, and preferences. There’s unfortunately a lot of differences from device to device, and it can sometimes take some experimentation to figure out what’s going on with yours.
Roon has no software volume control internally–in all exclusive mode configurations, we are passing on volume commands to the driver and letting your device manufacturer implement volume in the way that they prefer to. By disabling volume or forcing maximum volume in Roon, you are not changing the audio stream that leaves Roon in any way–you are just hiding the volume slider from the app.
I think “force maximum” should be reserved for solving specific problems when disable volume has proven insufficient for some reason (for example: you might run another app that leaves your device volume at <100% by accident frequently and you want Roon to put it back to 100% at startup).
Some devices implement volume controls in a suboptimal way–often as a digital volume stage prior to the DAC. In these situations, it’s a quality/convenience tradeoff. On these devices, if the volume level is 100% then the output is bit-perfect, and at <100%, it isn’t. You may choose to keep the volume slider in Roon for convenience (knowing that at <100% it’s not bit-perfect, but accepting that sometimes it’s nice to be able to reduce volume quickly from the computer), or you may decide that you’d rather hide the slider from Roon using “disable volume”.
In more complex hifi installs where the DAC and amp are in separate boxes and you want to use the amp’s volume control, it is a good idea to choose disable or force max, since whatever the volume slider is doing in Roon, it’s probably not communicating with your amp