We often recommend placing the Comment Card on any pages where you feel it will fit within the context of the choices you provided within the "Focus of Comment" question of the Comment Card.
For example, if you are using the Comment Card to gather general feedback with regards to your overall website, then we often recommend placing the script on every page within your website to provide a consistent experience. You would also be able to customize the "Focus of Comment" question within the Comment Card to show more general categories (e.g. "Site Navigation", "Amount of Information Provided", "Technical Issues", etc.).
On the other hand, if you are using the Comment Card to gather feedback with regards to a specific aspect of the website (e.g. for a Shopping Cart feature), then you could place the Comment Card on only pages which would be within the Shopping Cart / Purchasing process. As such, you may want to customize the "Focus of Comment" question to reflect the Shopping Cart feature accordingly (e.g. "Clarity of Pricing Information", "Ease of Navigation", etc.).