If your lights and objects are both not moving (i.e. only the camera is moving) you can leave photon map rebuild off and it "locks" the photons in place... also goes faster...

FG, however... even with rebuild off, it will still rebuild parts of the map... FG is based on the camera view, so when the camera view changes, it automatically generates whatever part of the map is needed to fill in the rest...

With rebuild turned on, it just rebuilds the whole map... I believe having rebuild off will go faster, but I am not sure by how much.

Now, as for the spots in your images... they look like they could be 3 possible things, or all 3:

1. You need to tweak your photon settings: try increasing the radius and accuracy til the "spots" dissapear. It just looks like the photons aren't overlapping enough to make a smooth "color."

2. FG needs to be tweaked, which is whats causing some noise in your shadow areas... I am certainly no expert on settings for FG, but generally speaking with FG rays set below 1000 you need to really mess with the radius settings to get a smooth look... to be honest, since I hate tweaking and re-rendering, I usually always start with only 100 FG rays and a min radius of 10, with a max radius of 100... usually I don't have any noise, but I do get some slight banding in the shadow areas... but its pretty fast, definately WAY faster than having 1000 rays.

Now, as for photon lookup, I always have it on when I use both photons and FG... in Mental Ray, this is a feature known as "fast lookup." For some reason, Mental Ray for Maya calls it "precompute photon lookup." This title actually makes more sense, as I'll explain in a second, but, if you have "progress messages" turned on when you render, you'll see it still calls it "fast lookup" in the output window...

Anyway, what photon lookup does is incorporate some preliminary FG calculations in the photon mapping stage... by doing this, it stores some FG information into the photon map iteself.

The end result is faster FG calculations, at the cost of slightly slower photon map calculations...

Also, this means that the more photons you have, the less FG rays you need.

I once had a scene with I believe 17 lights, all casting 5000 photons a piece, and I only needed to use 10 FG rays to get a smooth look. Anyway, moving on...

3. Sampling quality means a whole lot in Mental Ray... First off, the contrast values work in the opposite way you might expect. The higher the values, the LESS the quality...

.1 for all the values (RGBA) is the default, and works fine, but I pretty much always set it down to .05 for all of them... takes a bit longer, but not drastically longer.

Any lower than .05 and the quality improvement is negligable compared to speed loss.

Now, the next sampling quality issue is the min/max samples... generally speaking, you want to keep those numbers two apart... so if the min was 2 the max would be 4... if ithe min is 0 the max would be 2... etc...

now, a value of 1 2 isn't bad, persay, its just that it doesn't give mental ray as much "leeway," and can result in slower renders...

the lowest setting recommended is -2 0. You can go lower, but quality suffers a ton if you do, and speed does not improve all too much...

the highest recommended is 2 4... quality really doesn't get much better after that, and it slows down a whole lot.

For production-quality, 2 4 is recommended, BUT, 2 4 takes FOREVER to render...

Anyway, your setting of 1 2 isn't bad, but if you turn it up a bit to say... 1 3 or 2 4 a lot of noise issues will probably go away... just be prepared for terribly long renders.

Seriously, though, I think that most of the problem comes from the photon settings... I don't know how big your scene is (as in how long, wide, high in maya units), but basically, the larger the scene, the bigger the photons need to be to compensate... or you can add more photons, but it seems like you have plenty...

Anyway, the renders are looking good, noise issues aside : )

Hope I helped.

-Steve