Caching javascript variables

Question

I had code where I add some height to the div:

$('#some_id').height($('#some_id').height() + 100);

I was criticized for this by the employer, who said that I should not use the selector twice. I should cache the first time, doing something like:

 var el = $('#some_id');
 el.height(el.height() + 100); 

I assumed that there would be only a small saving if I cached the items selected using id, so I never worried. I'm right?