C.7.1- If it is illegal to attack or block with the creature, then the "must attack" or "must block" can be ignored since it cannot be followed. See Rule C.3 to see what makes a creature unable to attack, and Rule C.4 to see what makes a creature unable to block. [D'Angelo 1998/02/03]
C.7.2- If there is a cost to declare the attacker, the fact that it would be required to attack if possible does not require you to pay the cost in order to allow it to attack. For example, you don't have to pay the Brainwash cost on a Juggernaut. If you do pay the cost, it must attack, but if you do not, it does not. This is true even if the enablement cost is zero, as with Dark Maze. [D'Angelo 1997/10/13]
C.7.3- A creature that must attack on a turn, must only attack once that turn. It is not forced into every attack on that turn. [Duelist Magazine #17, Page 48, 1997/06/01]
C.7.4- If you have any creatures that must attack, then you must declare an attack. [D'Angelo 2002/03/16]
C.7.5- If a creature is required to block more creatures than it can legally block, then the defender chooses which creature(s) to block, but must choose to block as many as possible. [D'Angelo 1999/05/01]
C.7.6- An attacking declaration is not legal if a creature without "must attack" attacks thereby making a creature with "must attack" unable to do so. You must include as many "must attack" creatures as possible before including any creatures without it. [CompRules 2003/03/15 - 500.2]
C.7.7- A blocking declaration is not legal if a creature without "must block" blocks thereby making a creature with "must block" unable to do so. You must include as many "must block" creatures as possible before including any creatures without it. [CompRules 2003/03/15 - 500.3]
C.7.Ruling.1- You are not forced to maximize the number of "must attack" (or block) creatures that you can declare. [WotC Rules Team 1997/08/05]