Generally speaking I do not use my letters after my name; most correspondence does not call for this level of formality, and to use them in relatively common correspondence or indeed most formal correspondence smacks of pretentiousness.
However they may be used in appropriate academic settings: such as in the footnotes of an article. Also if one were to receive a formal invitation where the inviter has deployed such postnominals, it would be good form to reciprocate with equal formality. Listings of partners/tenants sometimes use this approach, as do business cards on occasion (although this is more infrequent than before). Hons, is in turn less frequently used since the majority of bachelors' degrees are honours degrees, and it is taken as read; indeed the usage of (Hons) in addition to the University name together often looks most clumsy.