For example, I have a website which has a dynamically-generated segment of its dom which looks roughly like this:

<div id="master">
  <div class="child"> ... </div>
  <div class="child"> ... </div>
  <div class="child"> ... </div>
  ...
  <div class="child"> ... </div>
</div>

There might be hundreds of child elements in this manner, and they, too, might have yet more children. At present, none of them have id's except for the master. Javascript manipulates them via dom-walking. It's a little scary.

I'd like to add id's, e.g.:

<div id="master">
  <div id="child1" class="child"> ... </div>
  <div id="child2" class="child"> ... </div>
  <div id="child3" class="child"> ... </div>
  ...
  <div id="childN" class="child"> ... </div>
</div>

What are the tradeoffs in adding id's to these divs? Certainly, jQuery must maintain a hash of id's, so there's going to be some sort of performance hit. Any general guidelines in when adding additional id's is a bad idea? Is jQuery so awesome that, in practice, it doesn't matter?