Importance of phase and timing optimization when incorporating sub woofers ?

No it could not possibly matter less.  

This is a hard one for people to grasp, probably because this is so critically important to getting good sound in the midbass, midrange and treble. But the physics and psychoacoustics of really low bass are so much different its important to understand. There’s a lot of guys who should know this but it seems to be one of the hardest lessons in all of audio, as you can already tell. If you really do want to understand there’s great info by Tim and Duke and me, and you can read Geddes, et al for more fundamental research. But I will explain briefly here.

First off, tests show human beings can’t even hear low bass of less than one full wave. The waves at 20-40Hz are as big or bigger than your room. So that right there should make it obvious there’s no point trying to optimize timing.
The fundamental problem with low bass is it bounces around the room creating zones of high and low pressure called modes. Room dimensions and where you put the sub in the room determines where these high pressure zones or modes will be. Moving the sub around will only move the modes around. Changing or shifting the phase is hardly any different than moving the sub. EQ can flatten the volume difference but its important to understand this happens across the whole room, when in reality its only the loud mode area where you want it less.

Two subs helps but not the way most people think. Two subs helps if they go in different locations. Because the more modes the smaller they are and the smoother the bass will be. For this reason four is even better than two.

If you actually do this, as I have, you will find that with 4 (or 5) subs it hardly matters where they go, the bass will be excellent. Superb. Better than you ever heard. In your life. Anywhere. Read the Swarm/DBA threads. You will see.

At low frequencies, speakers + room = a "minimum phase" system. In English, this means that the frequency response tracks the time-domain response. 

So if you have an in-room bass peak in the subwoofer region, you will also have modal ringing at that frequency - the energy will there will take longer to decay into inaudibility. (It is the frequency response peak that we actually hear; the ear as very poor time-domain resolution at low frequencies but arguably has heightened resolution in the loudness domain.) The good news is that when you fix one (by EQ or bass trapping or some other technique), you simultaneously have fixed the other.

Therefore, in my opinion, the phase adjustments on a subwoofer amp (and/or timing adjustments within a bass management system) are tools to be used with the focus on getting the frequency response smooth. Smooth bass = "fast" bass. In other words, don’t sacrifice frequency response in pursuit of what you think would be better phase response. In the bass region when the frequency response is right, the time-domain response (which includes the phase response) is also right. 

Duke

No it could not possibly matter less.  

Man who constantly argues against science completely misunderstands the problem, writes nonsense in long form.

JL Audio has a lengthy white paper on this subject; interesting read. Check out their website for another viewpoint, and one where the author does not insult the audience.

Adjusting phase will smooth the FR around the crossover frequency. The effect is noticeable by ear and it is measurable. (It is best done with a measurement mic.) Without such adjustments, one may have large dips in the FR due to cancellations.

As Eric said, adjusting delay and phase are essentially the same thing.