That’s probably a good practice anyway. /bin/sh is guaranteed to exist, but there’s no reason to assume bash is installed at all. If it is installed, /usr/bin, /usr/local/bin, /opt/bin are all reasonable locations.
POSIX explicitly says "Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH [...]"