Andy,
It's not the way it works. You are not introducing a negative, you are giving them a very strong positive.
Most of my friends run businesses in a variety of markets.
They all have it in place and many times apparently, it has been the difference between getting a sale and not.
When asked why they chose them, the customer mentioned the guarrantee. They said the competing companies said they never make mistakes but their (the customers') approach was that "there is a first time for everything". At least the guarantee gave them piece of mind.