One more thing, mostly about foreign words. I use a bunch of them (though less, since I've learned better), and if I italicized them all, it would read funny because we tend to stress italicized words when reading. So, while technically I should put them all in italics, from a readability standpoint it doesn't work so well. Probably the best example is the honorific on a Japanese name--no, it's not in the English dictionary, but putting it in italics tends to give it an emphasis it shouldn't have.
I guess my point is there is a set of rules, and you make the best decision for your work based on those rules.