Whenever I read threads on other forums from rookies seeking advice after a not quite satisfactory honing job, there always -usually another rookie- popping the question: "Is your hone lapped?".
In my experience, it is an overstated non-issue. Some synthetic hones glaze with swarf, and they need to be cleaned by lapping, that much is true. Equally true is that a severely dished hone will not create the best possible edge. But it is not necessary to be anal about it. Ardennes, for instance, laps their hones to a visual degree of trueness. If you do the pencil grid test on them (draw a pencil grind on the surface of the hone and check if you can remove the entire grid with just a few lapping motions on a known flat abrasive surface), you'll find that Ardennes doesn't lap them that flat. I guarantee that it doesn't make any difference. I have honed razors on Coticules that were even visually slightly out of true, and there was no problem whatsoever getting excellent edges.
So, please don't waste precious Coticule by lapping your hone to mathematical flatness, every few razors.
What I do recommend are a few habits to keep your hone flat within reason during its entire lifetime. (Indeed, it may never require additional flattening):
1. While raising slurry, spend extra time on the far ends, notably the corners of the hone. Secondly, concentrate on the "high" spots. They can be identified because you'll feel less resistance rubbing over them. At the lower spots, the rubbing stone will suck itself to the hone.
2. If possible don't use the hone in the same direction during the regular slurry stage of honing. Once you arrive at the "refining" stages, do keep the stone oriented in the same direction. (You don't want slight variations introduced in you honing at that point)
A Coticule is not a synthetic hone, but I doubt even those need the kind of frequent lapping that you see often recommended. I am sure manufacturers don't object against customers lapping their hones through the sink...
When this thread has taken its course, I'm going to distill a FAQ article to put in "the Mine" section.