I would put the number in front of everything else, always. Can't really see a downside to doing that.
As namida said, it's the "object obscuring terrain" thing which I got in trouble for when I made the lizard sit "on top" of the terrain in one of my levels. Both namida and kaywhyn mentioned that I should have made it "no overwrite" so that the terrain would not be obscured (incidentally, I noticed that your level "Keep Off The Grass" has a lizard-on-terrain
albeit off to the side where it probably wouldn't interfere with playing the level).
However, I think this should remain open for debate. I personally don't mind objects overlaying terrain; for one thing, it can make certain aspects of levels more aesthetically pleasing (such as the lizard thing), and the thing Strato mentioned about overlaying pickups over terrain is also valid: it's a good way to make them inaccessible without use of certain skills.
Another more classic example is water-on-terrain, which is used so frequently that it needn't be regarded as an issue at all (unless it's "
only on terrain, and the trigger area extends beyond the terrain, effectively making it "invisible water").
Ultimately, it comes down to whether we would prefer objects to
always be displayed behind terrain (meaning the "number-always-visible" suggestion is a no-go), or
sometimes allow them to overlay the terrain (meaning that "number-always-visible" is a possibility).
I'd also vote for it to be always visible, since mousing over the exit (as Gronkling suggested) might not occur to some players, and those who don't use CPM very often or don't necessarily take notice of the already-quite-large number of helpers at the beginning of the level may miss this detail, which IMO is as important as the skillset, time limit and any other item of level-specific data and should always be displayed.