You probably have an eyepiece cover that came with your DSLR when you purchased it… but are you actually using it? The answer is most likely no.
Should you use it? No. Not only is that eyepiece cover clumsy to take and remove, doing it too often has a change to loosen the eyepiece mount, making your regular eyepiece easier to fall off and lose accidentally.
However, it doesn’t mean that you shouldn’t cover your viewfinder at all. You can use a microfiber cloth, a cardboard or even by cupping your hands to cover the viewfinder in a pinch. While most cameras are well built enough in regards to light leaks, there are still some situations where covering your viewfinder might be a good idea.