Fitness tip: Don't underestimate the importance of having a sturdy and even foundation. It WILL give you significantly better results.
In this instance, I am talking about what your lower body looks like while you are performing any exercise that requires you to stand. A good example is a shoulder press. The closer together your feet are, the less stable the base will be. I typically like to stand with my feet shoulder width apart.
I see many people standing with their feet right together, or completely crooked and uneven as they believe that this will allow them to train their core harder. This fallacy will actually jeopardize their results as they will not be able to lift anywhere near as heavy a weight, and their form will likely suffer. Consequently, they will actually be using the core significantly less, gaining less strength, and building less muscle.
As for the knees, you should never stand with them straight or hyperextended. Many people have a tendency of doing this. You should always be in an athletic stance with your knees slightly bent, feet about shoulder width apart, your glutes engaged, and core braced.
Most people would never build a house on a weak or uneven foundation as their common sense would dictate that this is incredibly ineffective and inefficient. The same concept applies to the body.