Part answer, part question
I'd say the answer is "it depends" on:
1) How time-sensitive the action is (you'd need to think that through at least). More urgency = more immediate processing. And,
2) How frequently you process your inbox. More frequent processing = tossing into "in" is more *possible* (not absolutely more effective, just more possible).
I'm puzzling over the same question, so I'd be interested in others' answers as well. I'm starting to lean toward the toss-in-box if possible side of things, unless the action is sufficiently straight-forward that processing is self-evident. For example:
"Gee, the waiting for item that's already part of an ongoing project came due. I'd better call to follow-up." Because the item is already part of an ongoing project, there's probably not much more processing required than to note ... Next Action: Call for status re delinquent waiting for item.
HTH. (And looking forward to further insights to follow from others...)