A simple phonemic transcription uses letters of the simplest possible shapes. This is a typographical principle: the simplest shapes are the most familiar, the most typographically satisfactory. […] The conventions and traditions of the IPA often allow, for the representation of a particular sound, a choice between two or more different letters. In these cases where such a choice exists, one of the letters will usually be found to be more romanic […] than the alternative. (Abercrombie 1964 cit. u Wells 2008).
The letter r may, when convenient, replace ɹ, ʀ or ʁ in the transcription of a language containing one of these sounds but not a rolled lingual r. (IPA Principles booklet (1949) cit. u Wells 2008)
Since the DRESS vowel can be unambiguously written e, an English transcription with the symbol e is simpler than one with ɛ. The same applies in those languages that have just five vowels (or just five peripheral vowels) […] Whether its quality is closer to that of cardinal 2 [e] or cardinal 3 [ɛ] is irrelevant. On the other hand, in languages that have a close-mid front vowel in contrast to an open-mid one (e.g. French, German, a Scottish accent of English), the two symbols e and ɛ must both be used[.] (Wells 2009)
[W]e use the symbol l for the lateral approximant of many different languages. But in reading a transcription of German you have to remember to make it very clear; in French clear, but not quite so clear as in German; for most kinds of English it is generally darker, and may vary in colouring according to position; in Korean it has a noticeably retracted place of articulation; in some languages it is dental rather than alveolar. We don’t want to be bothered with such detail at every relevant dictionary entry: it’s better to just learn and remember that such-and-such a language or variety has such-and-such a quality of sound. (Wells 2010)
Abercrombie, David. 1964. English Phonetic Texts. London: Faber & Faber.
Wells, John C. 2008. Simplicity in transcription. http://www.phon.ucl.ac.uk/home/wells/blog0809b.htm
Wells, John C. 2009. e and ɛ. http://phonetic-blog.blogspot.rs/2009/03/e-and.html
Wells, John C. 2010. aw, shucks. http://phonetic-blog.blogspot.rs/2010/01/aw-shucks.html