Because its too easy to put way too much stock finish in corners, be real " thin " when applying True Oil to lock mortises, and other nooks and crannies. But do seal them with finish. Just stretch the oil out so it does not puddle or pool in corners, or bottoms. You will not need to put as many coats of finish in these areas as you do on the outside of the stock. All you are trying to do is seal the end grain wood in the mortises, so they don't absorb water, oil, cleaning compounds, etc. during the life of the gun.
The outside finish has to repel water, of course, but also has to deal with sunlight, ( UV rays), wind, bangs, dinks, and dents, and you usually want to put enough coats on the outside so that you get a smooth, clear finish, matte, or glossy, as you wish, so that you can take pride in owning your gun, while enjoying the fine grain of the wood stock. A well finished stock will not only bring out the grain patterns, but also the small stress lines in the wood, or the fiddle back that seems to only appear when water is wiped on the stock, or the stock is finished in a good hand rubbed oil. Even gunstocks with relatively uninteresting, straight grain can have fiddleback, and that makes them an interesting piece of wood to look at.