How much customization of letters of recommendation is necessary? While I know that students are encouraged to "tailor" their applications to the particular school or program they're applying to, does the same hold true for the people writing letters of recommendation on their behalf? In other words, is it possible just to change the "addressee" portion, and use a greeting such as "Dear Members of the Admissions Committee," or is more personalization required?  

Just to clarify here, I'm referring to the "pro-forma" parts of the letter, rather than the actual content of the recommendation per se.
I have two levels of customization. Firstly, if the letter is for an internship at a lab, or an application to grad school or a job, I tailor the qualities that I emphasize and link those qualities to the specific job.

A second level of customization is if I have any connection to the institution (for example, when writing letters of recommendation for AT&amp;T Labs - where I used to work - I might mention this explicitly in order to convey that I understand the local culture)

This is above and beyond the usual pro-forma customizations for the addressee etc.