The simple answer to "when do I need to personalize my job-search communications?" is always. The hard part is knowing what communications need to be personalized and how to make them so.
Frankly, there's no excuse anymore for not addressing emails and cover letters to an actual person. Gone are the days when you had to find a contact by picking up a phone or hoping someone's listing was in the Yellow Pages. The Internet is an expanse of data that allows you to find a person in an organization and address your communications to that person. "Dear Sir/Madam" and "To Whom It May Concern" don't cut it any longer.