Isn't it in fact true that C=US in printable string format
is a different entry than C=US in T61 ?
I vote for the minimalist convention:
a) PRINTABLE STRING is a subset of T61 therefore
b) if the data can be encoded in PRINTABLE STRING then do so, else,
c) code it in T61.
Note that this convention has as a side effect the feature that
knowledge of the value allows one to predict the encoding and
hence allows accurate recomposition of decomposed signed objects.
Two-cents worth ...
John Lowry
PS: A really fun issue is how to represent all those T61 characters
which aren't obviously presentable using the US ASCII character
set.