Originally posted by M2358 If you look at the ground glass inside the camera, it typically will have light seal foam around it so that when the mirror flips up, it hits the foam. This serves two purposes: it reduces mirror slap vibrations so that a mirror lockup or mirror prefire feature may not be not needed in the camera, and it works in conjunction with the mirror to make a light-tight seal around ground glass, blocking stray light from the viewfinder window from reaching the meter or film.
The light meter on Pentax DSLRs at least is up by the prism. So mirror up will not prevent light from the viewfinder affecting the exposure. If i am using a tripod I always use manual exposure mode, taking a reading with my eye to the viewfinder. If there is strong sunlight behind me I will use my hand to shield the viewfinder during the exposure itself.