Always.
There are people who think bears are shy and will run away. However, just like dogs, there is always an ornery one in the bunch somewhere. I equate bears to pit bulls, only 10 times larger. If you're alone in bear country it's a good practice, because they do size you up. For both you and the bear (or other animals you might encounter) bear spray is a good plan. You don't want the bear killed either if you were mauled. Or, you turn around and there's a cub right behind you... It's not his fault. So, best for both...