The garage door opener doesn't have to be perfectly level. As long as the door clears in the full-up position, it is considered good. However, it is recommended that the opener rail be angled down slightly when the door is fully opened. This helps reduce friction and noise during the operation of the door. The inclined plane concept is also mentioned, which suggests that angling the rail down can provide certain benefits. It is important to ensure that the garage door moves freely and doesn't speed up toward the top or slam when closing.