Should a function calling promise and not returning anything be considered an error?

I have a one method that looks like this:

doSomething(){
     return somethingPromisy().then((blerp) => {
         // do something with blerp
         return blerp;  // Modified, of course
     });
};

Then I have another method that looks like this:

doSomethingElse(){
    stepOne();
    var x = stepTwo();
    var y = stepThree(x);
    doSomething.then((data) => {
        stepFour(data + y);
    });
};

I'm using mocha+chai+sinon to test this code, in particular, doSomethingElse, and then I want to make some assertions - but how can I guarantee that the promise will be resolved by the time I make assertions? I know one option would be to change it to:

doSomethingElse(){
    /* ... */
    return doSomething.then(...);
};

If I do this, then it's pretty easy to write my test because then it's:

return doSomethingElse().then(() => {
    someFake.lastCall.args.should.deep.equal(expectedData);
});

In my test and everything is fine. But should I be returning the promise simply for the sake of returning the promise? I don't actually care about any kind of return value from doSomethingElse - I only care that when I'm testing that the function in doSomething.then was called, i.e. the doSomething promise has been resolved by the time I do my assertions.

So what's the "best" way to go about this?