Is it a must to follow the rules of the religion to the letter?

Hinduism is a religion that doesn't have hard and fast rules. Instead Hinduism promotes recommendations for living life that are guidelines. The reason we have guidelines instead of rules is because rules have to change based on situation. If we are stuck with one rule that cannot apply in a situation, then the rule will have to be broken or rewritten. Guidelines on the otherhand are more flexible.

This use of guidelines is derived from the way the law of karma works. Karma delivers the consequences of our actions based mainly on the intention behind the action. For instance, let us take not hurting another person. Hindu guidelines recommend the use of ahimsa or nonviolence as a cardinal code of living. However, sometimes the use of violence is condoned and it does not create a bad karma even if another person is hurt. Such as, in the case of somebody protecting themselves or another in their care from a violent situation. In this case even if the defender hurts the offender, there is no bad karma nor good karma accrued by the defender. His intention was to protect, and if the intention is sincere than no bad karma is created. However if you hurt another person out of hate, malice or vengence then the full force of the negative karma will be accrued because the intention was evil.

You all probably know all this already. I believe your question is more directed to lesser issues such as elders or priest saying things like you must go to the temple on Fridays. You must wake up early and pray to God. There are many 'must' and 'must nots' like these. Some of these are good advice while others like you cannot wear black to the temple, or you must not keep an image of Lord Shiva in your house have no basis in Hindu philosophical truths or in the guidelines of living. Such rules are mere superstition and they can be duly ignored. Certainly no harm will come to anybody if you do not follow them to the letter. However, you should endevour to follow the rules of living that your parents and community set out for you as they were created for character development or with your safety in mind. These are rules such as coming back home before a certain time or cleaning up your room.