I think this depends on your point of view. I would say that if it was something that had to be done during a specific time-slot, just place this on your Outlook calendar (with a notification, if desired) and keep it out of MLO. That way your calendar will reflect that you are busy during that time, and you won't get any scheduling conflicts.
It also makes a clear line between your "appointment" or "meeting" and "todo" categories. I'm using the GTD approach to manage my work that advocates that all "hard landscape" items (things with specific scheduling requirements) be placed on the calendar. The only way you can trust your calendar is if every time sensitive item is on there, right?
Just a case of using the right tool for the job at hand.
peace,
Tormod in Stockholm