Hi James,
Having PM's log time at the Project level for simplicity is a fairly common practice (so much so that we made it an optional setting in our Enterprise Stopwatch solution), and yes, those hours (and costs, if you've set them up) to then "count" against the Project's actuals. You must first choose to allow time to be tracked at the Project level as a global setting within Workfront (noting that it might be "off" in your environment, by default). On the flip side of the coin, some customers prefer to only allow time to be entered at the Task level on everything (even a single project-long "PM Time" Task), preferring to have hours specified at the lowest common denominator, and noting that they still then do roll up to the Project level total actual hours and costs.
Regards,
Doug