I'm getting in the habit of letting the caller decide, something like the following:

    myAsyncFunc = function (param1, param2, callback) {
      var usingPromises = (callback === undefined);
      var returnVal = usingPromises ? Q.defer() : this;

      someFunction(function (err, res) {
        if (usingPromises) {
          err ? returnVal.reject(err) : returnVal.resolve(res);
        } else {
          callback(err, res);
        }
      });

      return returnVal;
    };

So if I call myAsyncFunc(foo, bar) it returns a promise and if I call myAsyncFunc(foo, bar, cb) it returns itself for chaining.