I have had seat posts that slide down despite being properly tightened. On a post that is not bottomed against anything, you notice at some point, but with a bottomed one, how would you notice? If the clamp somehow became loose and you rode the bike, the seat post would seem to be not moving, bout it would be "banging" whatever it is bottoming against.
For me, that alone is reason enough to raise te s.p. at least 5 mm or 1/4 of an inch. I would also make some discrete marking on it such as it is barely visible when at proper height. If the post slides down a bit, the marking won't be visible, thus an indicator that you should check the clamp.
(I admit the one should do a periodic thorough inspection that should catch such things, but, realistically, how many of us go checking every bolt with a torque wrench before each and every ride?)
My take on Andrew's answer is that using the seat post bottomed down may cause a "micro hammering" effect at the contact point, where the softest metal can get deformed a little. If the seat post develops a bulge product of this hammering, it may become difficult to remove from the frame (I've seen this happen on cheap steel bikes/posts) On the other hand, the constant hitting may make the threaded insert (where the bottle cage mount bolts to) go out of round, making more difficult to insert/remove such bolt. It may also dislodge or move it out of true, depending on bike quality/materials.
Forcefully inserting a pipe inside another that has a bend in it, may wedge it, also making it difficult to remove. Depending on materials, one of the pipes will deform more than de other. (I have played with this while DIY-ing things)
So, in the end, for ease of mind, I would "sacrifice" a few mm of travel and rise the post a bit. After all, the critical position is the upper one. (at least for me) The lower saddle position is often not as critical.