When it comes to good normalization practice, is it better to not denormalize when attempting to store aggregate data? For example, what are the implications of deciding whether or not to have a "YTD purchased field" in a customer table? I've read that the decision to include this type of aggregate field is based on the need for speed, and therefore may necessitate including it in the customer table. I would like to hear some comments regarding this.

FYI, thread #130522 was helpful with regards to some other normalization questions if anyone is interested.