I noticed Visual C++ 2010 (my current environment) and XCode both allow non-standard single-line comments in my C programs. Since these are both my target platforms, I assume it's fine to use it? VC++ 2010 doesn't support C99, so I'm limited to C89, though I know C99 added support.
Anything that works is fine as long as you don't care about the environments where it doesn't work. If you have no ambitions to compile your code in an environment that doesn't support one-line comments (presumably comments that begin with
// and continue to the end of the line), then it's fine.