You can view a few documented cases of letter writing - eg http://www.watchtowerletters.com/Home.html
Carl Jonsson wrote a series of very well researched letters concerning 607 bce - http://www.freeminds.org/category/4-watchtower-correspondence.html
Franz documents a few letters that branch committee members , district overseers etc wrote during the brief "glasnost" era after 1975. They all ended up in the large file markedB. Of course , since then , the WTBTS has become much more authoritarian.
Basically they don't like any inquiring letters and even the most innocent enquiry will be resented. Be warned that it is a risky strategy as the WTBTS will probably copy their return letter (if they decide to send one - often they don't bother) to your elders & you would be "outed" as an apostate or "independent" questioning one.