One can get away without doing many recommended things until the day you can't. Trying to remove a glow plug that is coated with a thick jacket of carbon build-up is no walk in the park.

Reaming glow plug holes is not very difficult and if you don't want to pay Kent's somewhat inflated prices I'm sure you can find something similar on ebay or in the hardware store. Buy a tee-handle chuck with it and you are in business. I do it every Fall and test the plugs as ROLLGUY suggests. Make sure they glow quickly and AT THE TIP not just on the shank.

Kent's main product is step-by-step instructions and special tools for novice maintainers. For those, I think his products are worth the cost.