As still lifes are set up in your studio or painting space, you've got control over the direction of light. Your options are the same as the light direction in landscape painting, except you can keep it the same all day long!
In the photo, the shadow cast by the flower moves position and changes shape quite considerably depending on where I placed the lamp. (The setup in the photo is a flower with its stem stuck in the spiral binding of a sketchbook which has two pages open at 90 degrees. The lamp is off to the right-hand side.)
Remember: a shadow should enhance the subject, not overwhelm it, and it's always an option to paint it in a lighter tone than it is in reality. There is no right or wrong choice, it's a question of playing with the light/shadow and selecting what you like best. (See also: How to Paint Shadows.)
