the main search defaults to relevancy, but if people click the categories from the front page, those results are returned by recently listed. So renewing does give you a boost for category searchers, if there are any out there. Except for the wedding category, which has been totally revamped and only shows a few pages of 'relevant' selections.
I don't know if I'd renew something that was in a search ad, as its already going to be at the top of the page a certain number of times anyway :P When I did ads, it was for stuff that didn't show well in searches, as there's no sense in paying for something that already shows up well. if that makes any sense.