How much detail you put into a UML diagram is typically directly proportional to the amount of time it takes to author the diagram. Sometimes it makes sense to spend more time on it, at other times it doesn't. The level of effort you put into it should be determined by the purpose of the diagram. On the lower end of the effort scale is the situation where you are sketching a process or relationship of your software to quickly communicate functionality or design details to other members of your team. This might happen during design meetings with one team member sketching something out on a white board or a piece of paper for others to see.
A technical lead or architect might spend a little more time refining a diagram and give it to a coder to implement the design. In such a case, it makes sense to spend a bit more time considering some of the details of the diagram. Whatever the diagram conveys to the coders doesn't need to be etched in stone, but the more consistent...