If you're casting, say, urethane resin into a silicone mold, and the object has very smooth surfaces, you don't need to use mold release spray all the time. With very smooth planes, if you load up the mold release, it'll just show up as imperfections on the cast anyway. While mold release can help increase the performance life of the inner surface of your mold, it's not necessarily required every time.
If you're casting more complex models, or very rough surface, though, you do want to make sure your mold releases your cast. A can of mold release nearby is handy for those times.