First, consider using the Rapid Task Entry dialog for entering all of your tasks. It's documented in the MLO User manual, I think around page 107. If you turn off the option (near the bottom of the dialog) for inheriting parent contexts, new tasks will receive whatever contexts you enter in the "context" box. MLO will remember what you put into the context box until you change it.
Second, if you add tasks by on-screen inserts into the inbox, consider assigning one or more contexts to the inbox folder. Any tasks created in that folder will inherit the parent's context(s)