To state the obvious... if you have set your startup to "last location" as Marko indicated and always close MC with the Playing Now Tab active (or a zone under Playing Now if you are using zones), it will always open to Playing Now as you desire.
If you want MC to open to Playing Now regardless of which tab was active, then you can change Tools > options > startup > startup interface to Playing Now. That option however wipes out your tabs from the previous session, which I gather is what prompted your original post.
It makes sense the default behavior should be to keep tabs between sessions regardless, or at least have a checkbox to enable that (like every web browser does that I am aware). That way, users can have MC startup in any location they want, whether it be Playing Now, Images, Video, Playlists, etc.... AND still preserve one's curated tab setup.
Perhaps JRiver will read this thread and add that option in a coming build. I imagine there are some who would not mind seeing Cloudplay or other streaming options added to the list of startup locations.
FYI.. the new Tab Lock feature is so that when you click on an item in the tree, it creates a new tab instead of replacing the active tab with whatever was clicked on in the Tree.