I'm surprised that everyone who has responded to my post has defended the conventional wisdom on charity giving. But surely one should either borrow money to do a life time worth of giving right away, or save and do all charity in one's will, or otherwise concentrate all charity giving to a single moment in time.
Given Robin's comments, I'm not sure anymore when is actually the best time to do the giving, but it cannot be optimal to do what many people actually do, which is to give a percentage of one's income to charity as one earns it. This point is very similar to the one Steven Landsburg made in one of his Slate articles, http://slate.msn.com/id/2034/, which was that a donor should give all of his contributions to one charity, and not spread them among several. The logic is almost exactly the same.