Masterkeying  n.   1. the planned destruction of security in exchange for convenience

Simply put;  The most secure system uses a different key for each lock, but this is not convenient.  The most convenient system uses the same key for every locks, but this option lacks security.  A well designed masterkey system finds the optimum balance between security and convenience.

The most important step in masterkeying is planning ahead.  It is easy to 'paint yourself into a corner'; then all of your hard work and expense may be compromised trying to extricate yourself.  First make a list of ALL of the doors and locks involved.  It is important to think ahead.  Do you plan to expand?  Are you skipping over locks to save some money?  Include all of these locks in you master plan. -include all locks you can think of then ask these questions:

Who needs access? (ask this question for each lock)
write this down -preferably in a spreadsheet

Do I need restricted / patented keys? (to prevent unwanted duplication of keys)
this is always a good idea

How often will I be changing (rekeying) locks?
is there a frequent turnover of employees / tenants?

Gather this information before meeting with your locksmith and you will be ahead of the game.