If you think of components as buckets, you put the sketch in then you do it's body. then if you want to use that body somewhere else you just pick it up and move it.
if you want to edit the sketch somewhere else you just do that.
If that components does not have a sketch in it and you want to edit the sketch you can't,
unless it is still a linked component (has the chain symbol on it ), if it's not link you can not edit the sketch, as there is no sketch.
Using a sketch outside of a component if it has no body and it's just a guide it don't matter that much at all.
also if the component, you know that you will use it again right from the start make a component and keep anything to do with it in the same places.
If the sketch is from a projection use the components origin to make the plane for it to go on.
if it's going to be use somewhere else, this save's problems in the long run.
if it's stay put it's not so important.
If at anytime you are not sure just ask any question