Speaker grills

Do most of you listen with grills on or off?
I cut holes in the grills where the drivers are. Then I can just leave them on.
Off, off, the only way to go, unless you have cats.
Off with almost all speakers. A few companies specifically recommend keeping grills on, and I do.
On. Always. Vandy 5As.
On. I think they look better. The sound difference is negligible.
Off! They are sound blockers.
On. Can't hear a difference and they look much better.
Depends on the speakers.
Off - big difference on every speaker I have owned! Like Mattmiller says - Sound Blocker!
I notice a big difference in the upper range. I have never been to a live venue where there were speaker grills. Grills are WAF and that is the only logical reason they even exist, in my book, which I am working on a movie now:)
Always off when playing music.

Grills on when not playing to help keep dust out.
Off when playing. On otherwise to help prevent yellowing of the PP woofer cones from sunlight.

Makes an audible difference to me in the treble. Infinity RSIIb.
Off when in use and on when not.
For Sennheiser HD600 headphones the answer is definitely without grills.
Off: When listening on the weekends! Or impressing fiends during the weak....
As a rule of thumb, most speakers sound better with the grills off. I have the optional grills for my Magico S5's & found this to be the case. However bare in mind some speakers have been designed such that the speaker grills are an integral part of the design. Two such examples would be the classic Infinity Renaissance series speakers & most of the current Avalon models. Ultimately you'd have to a-b test the speakers with and without the grills to know for sure.
Of course it should be mentioned that most grills are *extremely* transparent to the sound, so what doesn't make sense is that having no grills would subjectively make so much difference in the sound. But nobody ever experiments to see why a transparent grill is so bad for the sound. You could even make an experiment using a grill that has only say 1/5 of the number of strands of fibers so the grill is mostly open. Then it would be really, really difficult to explain, no? Same goes for headphone grills, they are bad news.
It is also not just the grill itself but also the grill frame that can interfere with the sound.
I build and tune my systems to sound right with grills on. Grills stay on for both aesthetic and protective reasons.

Various grill designs will inevitably attenuate various frequencies, especially the higher ones to some degree, but between initial speaker design and various things that one can tweak themselves easily if needed, having grill on has never been a practical problem for me.

I once refurbished the hood grills on an old pair of OHM Walshes in that the originals had become worn. These speakers had a particularly laid back presentation. I had to choose a fabric to use and I ended up using a loosely woven wool fabric (much different than thin but much more densely woven material originally used) from the fabric store in order to keep grill effects to a minimum. That also made it easy to stretch and form the fabric tightly and cleanly over the grill hood without having to cut and sew much.
Really depends on the speaker like most have already stated. When I had the original DYn C1's they sounded much better without the grills. When I upgraded to the C1 Signatures - well not so much but ever so slightly better without them. Now my Raidho's don't even come with grills.
Remember those thick foam grills on the JBL Century L100's? That type of grill definitely had to go.
Grilles on always. One pair of speakers does not allow removal, one pair specifically designed for grilles on, the other pair I prefer the looks and sound with the grilles in place.
+1 as to what Mapman stated.
Off for listening, makes a difference in the upper end on my Proacs
and because they look much much better (I like to see the speakers, am I the only one?).

On at other times, just for dust protection.
I think keeping them on render a more neutral sound, sometimes taming a overall to bright amp
using proac studio 140
I leave mine on my Verity Fidelio Encores. They take just an ever-so-slight hard edge off the treble.