It's more important to take still photographs than it is to shoot any other thing in the movie.
Distributors will not accept a movie without still photographs.
If you don't have stills, the distributor will not distribute your movie.
Which means that without stills you do not have a movie.
Make sure you take stills.
Still photographs are:
Actors in character doing scenes in costume on set and well-lit.
Still photographs are not:
Your friends goofing around for the camera.
Here's a secret:
nobody wants your "behind the scenes" stills. They don't go on the video box, they don't go on the 1-sheet. Nobody cares about them. You show those to your mom. She likes that picture of you giggling, with blood all over you, as you eviscerate zombies. Put that picture on your blog. Do NOT send that picture to your distributor.
Send the picture of you in character, eviscerating the zombie.
Imagine the pictures on the DVD box or on the poster at the movie theater. Those are the pictures your distributor wants.
Here's an awesome example of a picture we take all the time but that the distributor does NOT want. It's a picture of the slate.
Why do we keep taking pictures of the slate? Because the idiot camera man keeps forgetting to go from "movie" mode to "still" mode on his camera and takes a still of the slate instead of rolling camera. We've since fired him.
How many stills is too many?
We took about 1200 stills on (0801) Earthkiller and our rep was "still" not happy with our stills. (Get it? "Still" not happy? Hoo!)
Take stills of every scene. Take them during camera rehearsals. Or, schedule time for stills after each scene. The more action you can convey in the stills the better. Do it.
And it still won't matter.
Why? Because you need to take stills of your actors in front of a bluescreen or neutral background anyway. Like this:
Now here's a still which came about because the distributor ordered specific poses. That makes our lives vastly easier. We put the lead in the pose, we shoot in front of a neutral background, everyone's happy. Not that anyone used this picture for publicity purposes, but we gave them what they wanted.