I have a .357 Magnum revolver. Is it always necessary to use magnum primers in my gun?

No. Just because the gun is chambered for a magnum cartridge doesn’t mean that you have to use a magnum primer. Most propellants that are quicker than Accurate Arms #7 in rifles or Hercules Blue Dot in handguns rarely need a magnum primer. Use the magnum primers when using slower powders, and standard primers with the faster powders.