The stem on my bike is at roughly negative five degrees. If I were to flip it upside down, thus making it positive five degrees, might I be putting pressure on parts of it that were not designed for pressure?
Almost all stems (aside from a few very expensive carbon ones!) are designed to be mounted either way around. The pressures are in the same direction regardless of which way up it is. (they're just reversed)
They are designed to be used either way up, otherwise manufacturers would need to produce double the number of stems to create positive and negatively angled setups.
Out of curiosity I've just looked through a range of carbon stems online, and the vast majority of them are of an identical construction to their less exotic counterparts (complete with writing on both sides) There were a couple of stems that integral mounts for computers that didn't seem to be reversible. But by and large, they all appear to be designed to be used either way around.