Mylar tape wing seals - effect on wing performance

I tried asking this questionon on a different thread; it may have been too
off topic, but I didn't get much of a response:

I'm putting mylar control surface seals on the wings of a Mini Nimbus this
winter. At the moment I'm planning on fixing them to the finished wing
surface. The alternative is to rebate (recess) them into the wing. This
creates a smoother surface but costs more money and is harder to undo.

Anyone know of any research (or anecdotes) on the impact on wing performance
of leaving such seals proud of the surface? My assumption is the airflow
over the wing is pretty much turbulent at the wing/aileron join anyway, so
the impact should be minimal.

If the recommendation is to rebate, is it equally critical for the both the
upper and lower surfaces?