I actually ran into a problem with 'which' and a '.cshrc' file a week or two ago when doing some shell scripting on MacOS X

In my case, it was much more subtle, as which was written in csh, but that wasn't the shell I was using. So when it called the .cshrc, it overwrote PATH, and would report programs that weren't in my path when I was using bash.

(of course, which is a builtin under csh, so the odds of you calling the script under csh in normal use should be non-existant)

Oddly enough, there's a note about 'DO NOT USE "csh -f"' immediately after the shebang line, which fixes the problem, but I have no idea what other implications it might have.