First you should determine your reference level by properly calibrating your receiver with a sound level meter. Or you can just pick a volume level that's right for you. I personally have a compromise. I properly set my receiver with a sound level meter, but I leveled all channels to around 70-71dBs instead of the recommended 75-76dBs. In doing so I can turn my Onkyo's volume up until the display changes to "Ref" for reference level (really 82 absolute) and I can have a loud but not ear damaging, run everyone out of the room, volume level. The key here: experiment. Also note that I'm not sure about the Denon but with the Onkyo you can choose to display the volume as an absolute value (0 to 100) or as a relative value like the -20 to -17 you speak of and that's why my volume number above is different from yours.
Anyway, whatever you normally set your volume to is fine. Dialogue Normalization can be adjusted for IF YOU want to or not.
For example the DVD A Knight's Tale has a Dialogue Normalization of -1. To compensate, you would turn your volume up 1 notch to be back at YOUR reference level.
Another example: Air Force One (first release) has a Dialogue Normalization of +4 so to compensate you would turn your receiver's volume down 4 notches to be back at your reference level.
Like I said earlier though you can totally ignore this if you want. I personally have my volume set to a point where I hardly ever have to touch it. But with DVDs like those above, and many others, I do adjust for Dialogue Normalization. At my reference level a DVD with a Dialogue Normalization of +4 or more like Air Force One would be to much. However, for HDNet I don't adjust the volume because that +4 is badly needed and a true improvement. Of course a better improvement would be if they could raise the volume level without having to use the Dialogue Normalization technique.
P.S. Dialogue Normalization in the case of HDNet is quite affective I must admit though. Now you can leave your receiver volume at one level while watching Satellite and when you turn to HDNet Dialogue Normalization turns up the volume for you!