Copyright statements were abandoned (as a legal tool) as recently as 1989, oddly the first of March, rather than the first of the year. However, don't get too excited, since there was a progressively-widening gap when an omission could be "corrected." It was only six months in the '60s, as I understand it, but by 1989 (I can't find when), you had five years.
For comics, this isn't going to be an issue, since they're not expected to be on sale long enough for the correction to circulate. But for anything else, you need to check the same item as it went on sale five year later, in some cases.
The only other comment I might make is that, while individual issues of a series will usually get registered individually, publishers had the option of registering a year's worth of periodicals as a single book. I can't think of an example of where it's happened to a comic book, but it's certainly possible.
For an example of the other situation, though, where only some books in a series got renewals, search the Copyright Office database for the Fighting American books. You'll see that only a handful were renewed, for whatever reason.
Oh, and as to why DC and Marvel have all their renewals in order, bchat has half the story. But the other half is that they hired lawyers whose job it was to check the original registrations and then make sure the proper forms got filed every six months. There are lawyers who specialize in that kind of work, and they don't come cheap...unless you happen to have hundreds of renewals to keep track of each year, in which case it's certainly cheaper than doing it yourself.
