Depends on the circumstances. Generally speaking you'd leave a good young healthy animal to breed. However, you owe it to the deer to know as much as possible about the local population. Observation and local knowledge are critical. You do see runts, youngsters with poor body shape, malform heads and antlers. Knowledge of conformity and how to spot strong genes tells you which ones to leave. From a sporting point of view, the most prized are the mature bucks with strong carriage and heavy antlers. They've had their time and contributed to the gene pool. However, the responsibility of deciding who takes over once you've taken them is also yours.