Hi Lindsay, the main reason some folks have challenges with not denting wire which they are flattening with a chasing hammer, is usually a combination of the tool used and the technique used.
"Chasing" is the process of bringing the hammer down onto metal, and then quickly sweeping it to the side. When we use a metal flat head chasing hammer, often the hand twists, resulting in the edge of the flat head hitting the wire with some force, thus denting it. To remedy this situation, you can either practice bringing the hammer straight down and then off to the side, or invest in a half dome chasing hammer, and never worry about dents again!
Answer contributed by Dale "Cougar" Armstrong