There are five basic ways for supervisors to make decisions:
Decide by yourself, with no help or interference from anyone.
Decide by yourself, but first find out what others (your boss, employees, peers) think.
Discuss the matter with your boss, and let her lead you to a decision.
Discuss the matter with your team, and guide them to a decision.
Let your staff vote, and go with the majority.
None of these ways is always best or worst. There are times when each is appropriate.
In a given situation, the method you choose will probably depend on your background and experience, the nature of the problem to be solved or avoided, the consequences of the decision, the level of your bosss (or peoples) interest, and the amount of information you yourself possess.
A good rule of thumb: If a decision wont work without your bosss active support, find out what she thinks then either agree with her or sell her on your idea.
Another general rule: If a decision will directly affect your people, get their involvement in some way. They are much more likely to be cooperative and supportive later on if they have some voice in the matter. This is especially true when the decision will cause a change in their workload, their normal schedule, their usual procedures, their job security or the way they are evaluated or rewarded.