The general instruction is to always stick to the template.
Sometimes you will feel you need to add extra text to a standard canned reply to make it more personal, and to relate it to the customer's message. ONLY do it if it's necessary.
Example: If a customer mentions that her husband loved the product, you can add at the bottom of the email - btw, glad to hear your husband loved the product :)
If you add anything to customer's message, follow the rules below:
ALWAYS be nice.
NEVER let your anger, bad mood, message content (abuse, stupidity) to impact the way you reply.
ALWAYS keep as close to the template as possible. No need to change if you don't have to.