I found an interesting article on the development of the standardised so-called "idioms" of Romansh in Graubünden (in German) today.
It's only a short introduction, but it reminded me of the fact that the existence of five idioms doesn't mean that there are also five dialects—in the context of Romansh, it appears to be traditional to use idiom to refer to the standardised written languages (of which there are five) and dialect to refer to the spoken language of a particular community. And there are far more dialects than idioms; the language varies quite a bit from town to town, in some areas more than others. (For example, Putèr is apparently comparatively homogeneous.)
There were also some interesting tidbits of information, such as that the extreme (south-)west areas of the Surselva, Medel and Tavetsch, have initial tg- in their words for "house", like Engadine and central Graubünden, but unlike the remainder of the Surselva (and that many features characteristic of one area tend to pop up in local dialects of other areas, such as "Surselvan" au in Lower Engadine); that the Jauer spoken in Val Müstair is more similar to old forms of Vallader than current ones; that Putèr has an orthography that is more suitable to the pronunciation in 16th century than to today's (a "feature" shared by a couple of other languages; English and Irish come to mind, though I think Icelandic and Faroese orthographies are also in this category); and that the Sutselvan written idiom was initially devised for the Val Schons but later extended to Domleschg and Heinzenberg by the device of certain diacritics and orthographical conventions that let each area choose their appropriate realisation of the written supra-phoneme.