conditionalTask.js

// register someTask, which is either an array of 
// tasks or an empty array based on someCondition
grunt.registerTask('someTask', (function () {
	return someCondition ? [ 'submodules' ] : [];
})());

// someTask is then used multiple times throughout
// the rest of the Gruntfile.
grunt.registerTask('default', [ 'someTask', 'watch:stylus' ]);
grunt.registerTask('test', [ 'someTask', 'intern-init', 'intern' ]);
grunt.registerTask('build', [ 'someTask', 'stylus:compile', 'clean', 'copy:dist' ]);