BibTeX and biblatex/biber are not case-sensitive with regard to the string that denotes the entry's type. Hence,
@mISc, etc [16 possibilities in all...] are all perfectly equivalent.
The same goes, by the way, for the (lack of) case sensitivity of the names of the fields within an entry. E.g.,
AUTHOR are all equivalent as far as BibTeX and biblatex/biber are concerned.
What to do? Choose whichever capitalization method best suits your style. Of course, if you don't want to standardize on any particular capitalization method, that's OK too. :-)
By the way, if you foresee spending more than a trivial amount of time on building and updating bib files, I would recommend you find an editor that has routines/macros that simplify the pretty-printing, formatting, and sorting of entries in a bib file. E.g., the WinEdt editor has a set of nifty macros -- called BibMacros -- that provide some great functionality. (Other editors also provide such macros.) As befits a set of programmable macros, you can choose your preferred method of lower/uppercasing. Then, click on "Clean bibliography", and all entries will get their string settings standardized. And, should you ever decide to adopt a different capitalization method, all you need to do is change a couple of macro settings and run "Clean bibliography" once more.