shebang line is needed in the file and only if it's meant to be run as executable (as opposed to sh file.sh invocation. It is not actually needed by script, it is for the system to know how to find interpreter.
EDIT: Sorry for misreading the question. If the shebang line is missing or not recognized, /bin/sh is used. But I prefer being explicit about the interpreter.
Note, that this behavior is not universal, IIRC, only some exec* family function do that (not to mention different platforms), so that's another reason to be explicit here.
