There's a lot of confusion over this stuff.
You can file for copyright protection for either the song/piece itself, or the recording, or both. If you want to cover simply the composition, a work-tape version will do (as long as the finished version doesn't have major changes/revisions). For this type of filing, you use Form PA.
If you want to cover the master recording (a different copyright from that of the composition), then you must have a completed version, and you file Form SR.
If you want to cover both the recording and composition, you can use Form SR.
Arrangements, like song titles, aren't really "copyrightable," however, it's a bit of a gray area, so that's something that would get worked out in court, heaven forbid...
A few years ago, the Library of Congress Copyright Office went to a single form. I think things got a bit confusing, because they went back to separate forms (PA & SR). I think that's the way things still are, although I haven't filed anything recently (I too save some money whenever I can! see below).
As to filing "in bulk," each form can only hold so many songs, and technically, you're supposed to file a form for each individual piece in your catalogue. That does get expensive and time-consuming, though. What many people do is file a bunch of songs in a "collection," basically a medley of songs for copyright purposes; you put them all on one form (I forget how many, but I think you can do up to 5 on one form??), and then later, if you license or publish one of those songs separately, you "pull it out" of the collection by filing for just that song individually. The form used to pull a song out of a collection is called a Change Form.
As to the benefits of filing versus not filing, versus "poor man's copyright" self-mailings, etc., etc., I will just say that there is a reason that the LOC has this service and that people use it. YES, it is absolutely true that your work becomes "copyrighted" as soon as it comes out of your head and takes on a tangible, fixed form. But there are certain legal benefits to filing (I'm not going into it here, but there's plenty of resources out there-- look it up). So it's best if you do so, if you can afford to at least file your songs in collections.
That's how things work in the US. Many countries have similar systems and also international agreements; as do the various performance rights organizations, so filing a form with the LOC will never hurt you, but is definitely a benefit over not filing.
GJ