grilles on or off the speakers??

To answer your two part question:
1) On, (at least in my case)
2) Science (at least in the case of my speakers).

1) I keep the grills on my speakers, mostly for aesthetic reasons, as I think the speakers look much better with them on. (However, if I felt that they impacted the sound in any way, I would probably remove them.)

2) My speakers, the Eggleston Works Andra II's, have very thin grills, that was specifically designed to have very, very little to no effect on the sound. (The grill fabric is basically a single layer of very thin cloth, and is attached to a very thin metal frame, that attaches magnetically to the face of the speaker.) However, I can easily believe that a thick foam speaker grill could indeed have an affect on the sound. Therefore, I do believe that there is a science to speaker grill design.

My two cents worth anyway.

Almost the same thing with woman, mostly for aesthetic reasons:-)

On my Dynaudio C1's the soundstage is much better without the grills on but my Mission/Cyrus 782's it didn't seem to make a difference. Personally to me both look better without the grills

you're spot on--having the grills off does almost always make an audible difference. my own experience is that most speakers sound better with the grills off, and i note that a number of high-end manufacturers (e.g totem, revel) don't include grills as standard on many models. i recall a particualr instance in which a friend of mine could not figure out why his meticulously restored, just-serviced klipsch heresies sounded so bad. after fumbling around for awhile, we realized that the grills were incredibly opaque--thick cloth over dense wooden baffles--and that removing the grills made a huge difference, as if a veil had been removed.

There are no grilles on my ESP speakers. The fabric covering is permament, so comparison is not possible. An obvious choice by the speaker designer to have you not listen without the fabric.
I prefer the appearance of my N802's with the grilles on and that's the way I listen. There is a difference (slight) between clothed and naked but I've become accustomed to the sound with the grilles on. Someone else may hear it the other way.
My Triad System 3 speakers are designed to be used with the grilles in place. The drivers are staggered in depth on the baffle and the grille provides the tweeter with a certain amount of controlled directivity. They don't sound right at all with the grilles removed.
So, my answer would be specially designed grilles are science and not snake oil. OTOH some grilles provide an enhanced appearance without undermining the performance of the speaker which I would also rate as good design.