Yes they do tighten up on there own! And yes you can do damage to the valve seats and guides by having them too tight. If the valve wont fully seat then you will get blow bye causing loss of power if the exaust valves are too tight and backfiring through the TB if the intake valves are too tight as well as loss of power.
Just take your plastics off and adjust them! It's only about a half hr. process! Maybe an hour for your first time.