I follow Rule 2: Double Tap.

Do not concern yourself with little things like using your ammo in a limiting manner. The reality of being a lone or soon to be lone survivor is that replacement guns and ammo are everywhere. Zombies don't care about ammo, guns or even supplies, the walking dead pass on by all supplies. The well armed survivor dies all to often because their reservation or hoarding allowed a wounded Zombies to get them; combat this double with the double tap.

Do not wait, do not conserve ammo, and do not look away...double tap that downed zombie because the odds are better for you if you do.....