It is very much a subjective thing, the only real rule about turning it off/on is "does it sound good to you"?
Initially, I would leave Dynamic EQ on and Dynamic Volume off (which is the default setting post-Audyssey) just to let your ears get used to the new sound, as Dynamic EQ can sound very strange at first.
In general, Dynamic EQ should sound awesome with film content; the way to test out your system is with a good Blu-Ray or DVD (because movies are mixed to a consistent, specified "reference" standard) and see if everything has good dynamics and tonal balance.
For TV and music, you may find excess / exagerrated bass with Dynamic EQ as, again, these sources may be mixed "hot" or with non-standard reference levels. If this is the case, try experimenting with the "Dynamic EQ Reference Level Offset" value in the "Surround Parameters > Audyssey Settings" menu. You may find that offsetting your cable box / CD player by "-10db" or even "-15" will provide a more pleasing overall tonal balance.
What you may find (as ezatnova has) is that with certain sources Dynamic EQ just sound terrible, and you should turn it off in these situations.
Finally, you should experiment with Dynamic Volume either (1) if you find when watching TV that you are really annoyed by fluctuating levels between commercials and programming and (2) if you find yourself constantly jumping for the volume on the remote when watching a movie (e.g. you raise the volume to hear the dialogue and then the explosions start and the wife starts yelling at you to turn it down).
The best way to test it is to find a passage that is really bad in terms of big dynamic swings, for example if you are watching TV and it goes from the program to the commercial and gets super loud... just rewind the DVR and then replay that passage a few times with Dynamic Volume set alternately at "Midnight, Evening, Day" modes and find the "sweet spot" where you still preserve the dynamics but control the major volume swings. Most people find that the "Evening" setting provides the best overall effect, but you should play around with it.
PERSONALLY -- this is how MY setup works but you should really experiment for yourself:
1. with cable TV, I watch with Dynamic EQ + Volume on "Evening" the vast majority of the time. I also have my cable box offset by -10dB to control some of the bass bloat on badly mastered channels.
2. with music, I listen with Dynamic EQ only (Dynamic Volume really constricts music range) and have the offset turned to the max -12dB (on your newer '10 model you can actually set it at -15dB). I will occasionally turn on Dynamic Volume if I'm listening to music late at night and don't want to wake the wife.
3. for movies (Blu-Ray, HD-DVD, SD DVD) I leave Dynamic EQ + Dynamic Volume on at all times, and I have my Harmony programmed with "Day/Evening/Midnight" buttons so I can pick whatever I want on the fly. I leave the offset at "0" since everything sounds perfect with movie content. If I'm watching a movie with the wife I don't even bother with the "Day" setting, I just put it on "Evening" and then I don't have to mess with the volume and my wife doesn't yell at me to turn it down