Basically, a runner can run to another base whenever he wants

Good things a batter-runner or runner can do:

• Be aware of when he has to run to a base, and when it is an option.
• Constantly calculate the risk of making an out versus the reward of advancing to another base.
• Steal a base

Bad things a batter-runner or runner can do when he's at risk and not touching a base:

• Get tagged out.
• Get forced out, which can only happen if he has been forced to advance, or he leaves his base too soon in the case that a fly-ball is caught.