It may seem like a redundant task. You just finished painstakingly loading each and every cartridge you will shoot through your high dollar match rifle at an upcoming match. You have handled the cartridges through countless preparation tasks. Why add one more to the list?
How much is a win worth to you?
Some of us spend a great deal of money and vacation time heading to matches across the country. The extra twenty minutes it takes for me to run each cartridge through my rifle is nothing compared to the frustration of loosing a stage because of a stuck cartridge.
Ideally you will want to chamber check your brass after you resize and trim it. This is the stage where an out of spec case is most easily remedied. However cartridges really need to be checked after bullet seating as well. It is possible for seating stems to back out or a lack of attention to cause a faulty adjustment. I do spot check my cartridges with a bullet comparator during the seating process, but nothing gives me the piece of mind like feeling the bolt slip into battery on each cartridge.
When you chamber check live ammunition, you must observe some safety considerations. First, always keep your muzzle pointed in a safe direction. Second, it is highly recommended to remove your firing pin or firing pin assembly. This will make it almost impossible to accidentally drop the hammer on a live cartridge at your loading bench. Finally, keep your rifle on safe (if mechanically possible) and never place your hand into a firing grip or your finger near the trigger.
If you are doing this with a Remington 700, REMOVE YOUR FIRING PIN ASSEMBLY FROM THE BOLT!!! Do not chamber check ammo on a Remington 700 with the firing pin assembly installed.
If you find a cartridge that will not chamber, reject it. Determine if the bullet was seated long, primer was not fully seated or if there was a problem with the case resizing. If the case needs to be resized, pull the components apart. Do not attempt to resize a loaded cartridge.
One final, simple check can help eliminate frustration and embarrassment on the firing line!