eBay pretty much acknolwedged the flaws inherent in DSRs when they stopped low ones from giving defects to sellers (they didn't always, of course, but for a - thankfully brief - moment in time, a 3 or lower score for description, and a 2 or lower score for postage time / cost resulted in a defect that could affect the seller's account, or even see them suspended in enough were accrued. If memory serves, communication couldn't get a defect, even if marked a 1).
They are wholly subjective, and I've heard a lot of reasons why buyers mark down that I personally don't agree with, but one person's logic is often another person's irrationality (eg there are buyers who will never mark a secondhand item at 5, only brand new items, or those who rate on a scale of "like" rather than genuinely considering the accuracy etc of descriptions, postage quotes and all that stuff. I send a lot of stuff as large letters, and I charge less than $3 for this ($2 for stamps, 50c priority on all of them, and the padded envelope), yet I still get people complaining about cost and suggesting I should be using small letters to send for $1 instead. (Sometimes I flex my fingers and tell these people all the things wrong with that suggestion, other times I just ignore).
I've even heard of a buyer who complained to a seller over a 10c discrepancy in what they paid for postage and the stamped price on the package (so they didn't even think about packaging costs), so it's essentially impossible for every single person to know what's involved in all aspects of selling, to understand what they're being asked to rate, and then to do so objectively.
You also have some buyers who like to be communicated with directly, and other buyers who very much dislike getting even the automated emails, let alone additional ones from the seller, so no matter what your personal "best practices" are, there's simply no way you can work out a method that hits everyone's buttons, so to speak, and no matter what you do, there's likely to be at least a few who will dislike it and mark you down, even if the majority love it and leave glowing FB.
So, the best thing to do is to simply carry on with what works best for you, and consider the DSRs as an overview of people's (occasionally irrational) subjectivity rather than an accurate portrayal of your performance as a seller. I know that as a buyer, I won't even look twice at DSRs unless they are well below average, and 4.8s or 9's definitely aren't.
