With regard to exams, yes it can be difficult. I find that the marks of around 80% are completely clear, but I probably agonize over the marks for the other 20%, often reading the exams again. I find it much harder for paper courses than for exam courses (those that have hypos rather than essays). I make a really detailed marking key out of 100 potential marks and that helps with grading.
Yes, the line between two grades can be very close, but I would not say that it would come down to just organization. If two exams had the same substantive content but one was organized better than the other, I would not draw a letter grade line between those two grades. Sometimes the line isn't that fine and it draws itself naturally. For example, I might have an exam with 100 points and seven students had the following grades: 84, 82, 80, 78, 77, 76, 75. There is a natural gap between 78 and 80 where I would draw the B+/A- line. However, it would be much more difficult if the grades were 82, 81, 80, 79, 76, 76, 75. I might bump the 79% up into the As, since the exam answer quality is closer to those exams. I would seldom use this rule to bump people down though, unless it was required to adhere to the curve (ie if the marks were 85, 84, 84, 80, 79, 79, 78, I would probably not bump the 80 down).
With regard to the similarity between a 66 and 70, yes, they can be very close. That is really just a couple of marks. In terms of bumping a 66 up to a B, I don't think I've ever had this happen, but I've definitely bumped lots of 78s and 79s into the Bs where I had too many C+s. Perhaps this is what happened with the 66 that you are talking about. There may have been too many Cs, so almost all became C+ and almost all of the C+ became B. Different schools have somewhat different grading policies, but you have to be concerned with both making sure that you comply with the average that the school sets and you also have to have a distribution of grades. Sometimes it all works out where the grades sort of distribute themselves and other time it is really, really difficult.