The convention in puzzles where strict standards of clueing are applied is that using a lower-case letter to start a word which must be interpreted by the solver as a proper noun (eg eve when you mean Eve) is a no-no, whilst the reverse - using an initial capital for a common noun (eg Josh when you mean josh) in a place where capitalization is not otherwise required - is considered undesirable but acceptable. The latter is usually justified by an argument along the lines of "The name Eve can never legitimately be seen written as 'eve', but the verb josh can on occasion legitimately be seen written as 'Josh', eg at the start of a sentence." Of course, getting the ambiguous word to the start of a sentence solves the problems, but is not always possible.
As wintonian says, when it comes to abbreviations the rules go out of the window. 'R' is an abbreviation for River on maps (not 'river') and 'Rd' for Road (in addresses), but 'river' and 'road' are universally accepted. Oddly, the use of 'Stokes' in the middle of a clue to indicate S might raise a few eyebrows (the abbreviation is for 'stokes'), but it's actually 'less wrong' that king, queen, rector or river=R.