I heard somewhere (I don't remember where) once that a seat post clamp should be oriented with the gap at the front of the seat tube, opposite of the gap in the top of the seat tube. The logic behind this was that it supposedly exerts more equal pressure around the entire seatpost.
However, I've noticed that bikes off the rack are almost always sold the other way around, with the two gaps on top of each other.
Does it really matter? Is one way better than the other?
Typically, for an aluminum post you align the gaps. For a carbon post you turn the clamp 180 degrees from the gap in the seattube to minimize the chance of crimping the seatpost. Some manufacturers have their own recommendations, but they're typically in line with what the aforementioned guidelines.
According to a forumer working in bike building industry (SUNN, France), mounting the seat post clamp with its gap on top of the seat tube gap is important to ensure an equal and secure gripping.
Aligning the gaps will allow the clamp to compress the tube a little better, therefore reducing the amount of tension needed from the clamp to hold the seat post in. And less tension means a longer life for your seat post clamp.
Now, there's the issue of which way your clamp lever faces if you use a quick release clamp. Most have it facing backwards so it doesn't interfere with clothing and doesn't catch on anything while riding. But, with something this small and utilitarian, I think you can put it any way that keeps your seat post from falling and it's probably OK.
My two cents:
If you align the gap in the tube with the gap in the clamp, the grip will be much stronger. You'll need less force in the lever to hold the seatpost in place.
If you keep them counter-aligned, you'll exert considerably less pressure, and there is a chance that the pressure will be "more evenly" distributed, although I don't "feel" it to be necessarily like this, since it is only the frame itself that actually deforms to grip the seatpost, and I think it tends do deform the same way on both cases (a lot of common-sense here, that would need experimental data to be totally sure).
My advice would be: use first option UNLESS you have reasons not to do so. Such reasons could be: sensitive seatpost (carbon fiber, delicate metals), or a tendency from the clamp to leave marks/grooves in the seatpost.
It is not the clamp who bite into the post. The edges of the seat tube will. If you align the slit of the tube with the slit of the clamp the tendency of the edges will be to follow the easy resistance path which is a straight line and not a curved line. If the clamp is reversed, using a relative thick non deformable clamp, the edges of the tube will follow the radius and not crimp into the post. Of course the effort of tightening will be substantial cause now the clamp act to the whole section of the tube and not just to the edges
Reversed if the post is carbon, especially for an expensive light weight post. Apply a very thin coat of fiber grip, then you don't need to go beyond the recommended Nm force.
I do not think it is a very important factor to have seat tube gap and seat clamp gap aligned. What is important is that both are as perfectly round as possible. If for some reason either of the components (seat post included) are not round, the whole clamping system (seat post, seat tube and seat collar) is non-concentric (it is eccentric) and that is where the real world issues can arise.
It seems to me that based on the exact relative orientations of those imperfections (eccentricities), alignment between seat tube and seat collar gaps comes into play, but not for the reasons of gaps simply being non aligned.
That being said I do align my seat tube and seat collar gaps, partly because my seat tube gap is on the front, and that makes seat collars gap area a bit more protected from dirt and debris.
Please carefully scrutinise my answer since I am not an engineer.
