as always, there’s a variety of possibilities within the TEI Guidelines for the proper encoding of a specific feature ;-)
One solution could be to have a fixed set of terms (e.g. „postcard“, „letter“ and „telegram“) and simply put these as type values on the element <text>.
If you want more power to your taxonomy you should have a look at <textClass> (and its children)  as well as <classDecl> . One of the great advantages of this second approach is to be able to link to external classification schemes which can facilitate interoperability with e.g. library catalogues.
So, personally I think it’s a good idea to provide at least some classification but how much effort to put into this depends totally on your project demands.
Hope that helps,
Am 04.02.2014 um 19:22 schrieb mandy gagel <[log in to unmask]>:
> Hello All,
> I am hoping to get some feedback from those on this listserv who have experience differentiating between doc types of letters.
> I am currently helping with the digital edition of the Papers of Frederick Law Olmsted and this includes not only letters, but reports, essays, etc. Therefore, we are assigning <doc types> identifications to the header of each document.
> For his letters, there are personal letters and letters that he had published in a periodical (such as a letter to the editor).
> I am thinking of differentiating between the two types of letters, for example using <personal letter> or <letter> and <published letter>
> Has anyone else made this distinction in their projects and did it just make more problems for nomenclature, etc. or was it helpful?
> Mandy Gagel
> On Tue, Feb 4, 2014 at 12:36 AM, Peter Stadler <[log in to unmask]> wrote:
> Dear all,
> apologies for cross-posting but the following call for papers may be of interest to members of our group as well.
> Anfang der weitergeleiteten Nachricht:
> > Von: Martin Mueller <[log in to unmask]>
> > Betreff: First Call for papers for TEI Conference at Northwestern, October 22-24
> > Datum: 3. Februar 2014 20:39:57 MEZ
> > An: [log in to unmask]
> > Antwort an: Martin Mueller <[log in to unmask]>
> > Dear Colleague,
> > This is a first call for submissions of papers and posters for the 2014
> > TEI meeting, which will be held at Northwestern University, October 22-24,
> > 2014. Northwestern is located in Evanston, a northern suburb of Chicago
> > with excellent public transportation to Chicago's "Loop."
> > The deadline for submissions will be April 30, 2014. More soon about the
> > details of the submission process. As in previous years, we welcome
> > submissions on anything plausibly related to the Text Encoding Initiative,
> > but we have a special interest in the following topics:
> > 1. The TEI is about text "encoding," but encoded texts need to be
> > "decoded" by readers who put the encoding to various uses, increasingly
> > with the aid of digital tools of one kind or another. What is the
> > scholarly value added by encoding? What can people do with TEI encoded
> > texts (and what have they done) that they could not otherwise do or have
> > done?
> > 2. In 2015 some 25,000 TEI-encoded Text Creation Partnership (TCP) texts
> > printed before 1700 will be released into the public domain, and another
> > 45,000 texts will be released in the five years to follow, producing by
> > 2020 a deduplicated, structurally encoded, and open source library of just
> > about every English book printed before 1800. This is a very
> > consequential event for the documentary infrastructure of Early Modern
> > Studies in the Anglophone world. It is also an important event for the
> > TEI.
> > 3. As announced earlier, the TEI meeting will partly overlap and share
> > some programming with the Chicago Digital Humanities and Computer Science
> > Colloquium, a lively regional conference. Music will be one of its
> > featured topics and may be a good topic for shared programming. We will
> > welcome submissions about TEI and MEI in the context of music in a digital
> > world.
> > We have not yet decided on keynote speakers, and I would very much welcome
> > off-line suggestions about suitable candidates.
> > With best wishes
> > Martin Mueller
> > Chair, Programming Committee, TEI Conference 2014
> > Professor emeritus of English and Classics
> > Northwestern University