This depends on the application. In your example
[ must be quoted as an argument for
grep but not
For the shell (from the POSIX specs):
Quoting is used to remove the special meaning of certain characters or
words to the shell. Quoting can be used to preserve the literal
meaning of the special characters in the next paragraph, prevent
reserved words from being recognized as such, and prevent parameter
expansion and command substitution within here-document processing
The application shall quote the following characters if they are to
| & ; < > ( ) $ ` \ " ' <space> <tab> <newline>
and the following may need to be quoted under certain circumstances.
That is, these characters may be special depending on conditions
described elsewhere in this volume of IEEE Std 1003.1-2001:
* ? [ # ˜ = %
The various quoting mechanisms are the escape character,
single-quotes, and double-quotes. The here-document represents
another form of quoting; see Here-Document.
Specific programs (using regexes, perl, awk) could have additional requirements on escaping.