In Starting Strength, a well-known book on barbell training, Mark Rippetoe says (emphasis mine):
Grip width, within a certain range, is largely a matter of individual preference. Since you are trying to
develop general upper-body strength, your form should be generalized, without too much emphasis on any one
muscle group and with a lot of work for all of them. The greatest range of motion is obtained with a grip that
places the forearms in a vertical position when the bar is on the chest. With a wider grip, the bar doesn’t move as
far and locks out before the triceps have done much work, so the pecs and delts end up doing more of what work
gets done. But as long as the grip falls somewhere between 22 and 28 inches between index fingers, the purpose
is served. This range allows enough leeway for people of all shoulder widths to find a grip they feel strongest
with, while preserving the longer range of motion. Too much narrower a grip will, for most people, take pounds
off the work set by placing the responsibility for most of the lockout on the comparatively smaller triceps,although
most lifters can close-grip a fairly high percentage of their standard-grip bench press. A wider grip shortens the
range of motion excessively and takes out too much tricep, as well as producing a longer moment arm between
the hand and the shoulder joint. Heavier weights can be benched with a wide grip since the bar doesn’t have to
move as far (the legal width for powerlifting competition is 32 inches between index fingers).
Ideally, your spotter should tell you if your forearms are in a correct position when down; if that is not possible, then use whatever feels comfortable to you (but between 22 and 28 inches). Don't use a too wide grip if your goal is strength: the shortened range of motion is sub-optimal.