Just to add more info: Bear cannisters are necessary only for overnighters, but not to keep bears out. It's to keep the mice and marmots out of your food.
Bears have been a problem in the past, but only at Whitney Portal, for hikers getting ready to day hike while it is still dark. They will set their day pack down, and a bear will zoom in, grab the pack, and streak away. So beware what and where you set things down in those dark early morning hours.
