well, neither.... and both....
if you really understand what's happening there really isn't a conflict between the two.
the way it works is this:
1. Audyssey measures all of your speakers and creates EQ filters across their operating range, all the way down to their -3dB point (i.e. basically, the lowest usable bass extension). So, let's say your Nanosats are found to have a -3dB point around 140Hz or so.... Audyssey will create filters down to 140Hz but not below that, to avoid boosting a speaker in a range which it cannot really play.
2. Audyssey reports its measurements to the receiver, and the receiver picks the next higher crossover point. On the Denon, the two closest choices are 120Hz and then 150Hz, so the Denon (which "knows" that the -3dB point is 140Hz now) sets it at 150Hz.
3. The subwoofer is filtered over its entire operating range, typically up to 250Hz (which is the max crossover setting on the AVR). So, at a 150Hz crossover point, the Nanosats get the benefit of Audyssey MultEQ filters all the way down to that crossover, and then below 150Hz the subwoofer picks up (also being filtered by MultEQ).
4. The reason Audyssey does not recommend LOWERING a crossover is that you will now have a region which is unfiltered by MultEQ. For example, let's say you lower the crossover to 120Hz manually. Now, there will be a small section (120Hz to 140Hz) that will fall outside the bands of Audyssey's filters; the Nanosats roll off at 140Hz but the sub doesn't pick up until the crossover of 120Hz. That band from 120-140Hz will be a "hole" in the Audyssey filters.
Thus, Audyssey as an "official" position recommends not lowering the crossovers so that you (1) don't ask the speakers to play frequencies they have been measured not to be able to in your room and (2) don't have a frequency band that is unfiltered by MultEQ.
Now, on a PRACTICAL level, which is where I'm coming from, the fact of the matter is that these crossovers are picked by the receiver and may not be the "optimal" crossover in terms of creating a smooth response with your system. So, taking the above example, you might find that TO YOU things sound better, with a smoother blend between sub and satellites, at a 120Hz crossover. Bass above 100Hz becomes progressively more localizable and creating a seamless blend between tiny satellites and a subwoofer can be tricky.... so, you may find that it's the "lesser of two evils" to lower the crossover slightly, such that it's worth having a small "hole" in the filtered response in order to create a smoother blend in your room between subwoofer and satellites.
The situation becomes even more extreme with a super high crossover like 200Hz. That is the next higher step up from 150Hz in the Denon's crossover settings.... so, let's say that some of your speakers are rolling off at 140Hz, some at 160Hz (maybe due to room acoustics, how close to the wall they are, etc.). The ones that roll off at 140Hz will be set to 150Hz crossover by the receiver... but the ones that roll off at 160Hz will default all the way up to a 200Hz crossover. Again, lowering that crossover to 150Hz would "technically" be against the Audyssey recommendation, but it may sound a lot better to have a tiny (150-160Hz) band that is uncorrected considering the tradeoff of not sending all that localizable bass (150-200Hz) to the subwoofer.
So, while the "official" position in the Setup Guide has to be "don't lower the crossovers" in order to avoid confusion and gray area... on a practical level, lowering the crossovers one notch might end up improving the system to your ears. I would NOT lower the crossover drastically -- for example, if you are getting 150Hz as the set crossover do NOT go all the way down to 80Hz! Those tiny speakers are rolling off well above that and you will create a big hole in the response of your system. But a SLIGHT change downward, e.g. trying out 120Hz, may end up working out as a net positive. So the "batpig recommendation" is, if you end up with a very high crossover (like 200Hz) go ahead and try out lowering the crossover slightly, and let your ears be the judge of which sounds better.