In the vast majority of scenarios, you can keep your conservation of mass separate from your conservation of energy. You only have to pay attention in very specific cases: fusion, fission, and relative velocities that are quite intense.
This refinement, which we see again and again in physics, is "oh, sure, if you do THIS, here's a correction you can apply ... should start showing up three or four sigfigs in."
In the vast majority of scenarios, you can keep your conservation of mass separate from your conservation of energy. You only have to pay attention in very specific cases: fusion, fission, and relative velocities that are quite intense.
This refinement, which we see again and again in physics, is "oh, sure, if you do THIS, here's a correction you can apply ... should start showing up three or four sigfigs in."