As the converse, if you don't know what's in the POSIX spec, or haven't tested widely, please put "#!/usr/bin/env bash" at the top of your script. Most people who don't have bash as the default shell at least have it somewhere. And those who don't would rather have your script fail with an obvious error than a subtle one.
If you don't know what's in the POSIX spec, you should go and read it. If you do this, you will learn not only what the shell must do, but also that /usr/bin/env might not exist at all. The env utility is required, but it isn't always in /usr/bin.
