I read this thread in bits and pieces over its course and don't remember everything that was said, so I apologize if I've repeated something.
erase: Some people also never have trouble with it because they learned to draw both ways from the beginning. Skew is almost inevitable at the beginning, but heads facing in either direction are susceptible to that. I was/am also ambidextrous, but I learned to draw in a rather... flawed way, so for a while I could only draw heads facing one way. At one point, I switched styles and started drawing heads facing the other way, and could not draw the first way any more (I always drew with my right hand then). So, I don't think handedness is as related to it as it seems; I think it's a habit that happens to arise based on handedness, but like krazy said, it's one that can be broken with practice.
I don't think that the reading direction has to do with how an artist draws heads, as any comic will have characters facing in all kinds of directions (or maybe 3/4 one way and the other, one of which may be drawn worse than the other, in the case of really limited artists). Correlation does not imply causation; I think the issue is more that there are a lot of artists who draw right to left who can only draw heads in one way, and who are just not good in general, and are right-handed because that's the majority... I am sure that if we look, we'll see the same proportion of left-to-right comics done by righties who draw heads facing the same way as the R-to-L artists. Maybe not as many by count, but that's because the current crop of young artists are heavily inspired by right-to-left manga because the market is inundated by it.
Most people seem choose right to left for these reasons, or a combination thereof:
1. It's what they're used to, because they mostly read manga
2. They want to emulate manga because their comic is "manga" (this is a crud reason)
Most of them do not consider the written language at all, and.. I find it hard to fault them because their audience grew up reading translated manga, so they can read the mismatched flow without being taken out of the comic. However, I think that this is an adaptation, they learn to jump back and forth on a page rather than allowing the page to lead them as the Japanese artist intended. In their own comics, they do not make use of that sort of flow either, but I wonder if that's related to this adaptation or just because they're not aware of the concept in general (many L-to-R artists aren't either). It is that natural flow that, for me, makes the difference, and it is why I make my comics left to right despite 95% of my comic reading material being manga, and why I prefer to read R-to-L comics with R-to-L text, and L-to-R comics with L-to-R text.
Also, there are some L-to-R comics drawn by Japanese artists who normally draw R-to-L (Ikaryaku and Nihei Tsutomu's Halo comic come to mind). They look and read just fine. I think this is because the artists know how to draw a comic well, can draw anything in any direction and make use of directional cues to guide the reader. For this reason, I think #1 above is a BS reason - an artist who knows what they're doing shouldn't be bothered by the direction their comic goes in, and should be able to choose their direction so that their intended audience can read it in the most effective way. The reason it does bother a lot of artists is probably because they don't think about their comic's structure enough, they just draw what they're used to seeing instead of dissecting it and figuring out why they should or should not use it. In short, I think a comic's reading direction should be whatever tells the given story best to its intended audience.