As of Jan 1st 2012 the rules section for 1.4 will be updated to include this.
1.4. OPTIONAL ABILITIES
TRADITIONALLY, SOME ABILITIES INCLUDE THE WORD ‘MAY’ AS PART OF THEIR TEXT, INDICATING THAT THEIR EFFECT IS OPTIONAL. AT COMPETITIVE AND PROFESSIONAL REL, SOME ADDITIONAL TRIGGERED ABILITIES AND ENTERS-THE-BATTLEFIELD REPLACEMENT EFFECTS ARE CONSIDERED OPTIONAL. THE PLAYER IS NOT REQUIRED TO FOLLOW THE INSTRUCTION WHEN THE ABILITY RESOLVES, AND IF THE ABILITY IS FORGOTTEN IT WILL NOT RETROACTIVELY BE APPLIED. AN OPTIONAL ABILITY DOES ONE OR MORE OF THE FOLLOWING THINGS, AND NOTHING ELSE:
• GAINS YOU LIFE OR CAUSES AN OPPONENT TO LOSE LIFE.
• PUTS CARDS FROM YOUR LIBRARY, GRAVEYARD, OR EXILE ZONES INTO YOUR HAND OR ONTO THE BATTLEFIELD. THIS INCLUDES
DRAWING CARDS.
• CAUSES OPPONENTS TO PUT OBJECTS FROM THEIR HAND OR THE BATTLEFIELD INTO THE LIBRARY, GRAVEYARD OR EXILE.
• PUTS A PERMANENT INTO PLAY UNDER YOUR CONTROL OR GIVES YOU CONTROL OF A PERMANENT.
• PUTS +X/+X COUNTERS, OR COUNTERS LINKED TO A BENEFICIAL EFFECT, ON A PERMANENT YOU CONTROL.
• GIVES +X/+X OR A BENEFICIAL ABILITY TO A TARGET CREATURE YOU CONTROL.
• EXILES, DAMAGES, DESTROYS, TAPS, OR GIVES -X/-X TO AN OPPONENT’S TARGET PERMANENT. IF THE ABILITY COULD TARGET
YOUR OWN PERMANENTS, IT IS NOT OPTIONAL UNLESS THAT ABILITY COULD TARGET AN OPPONENT.
• GIVES YOU ADDITIONAL TURNS OR PHASES.
• COUNTERS A SPELL OR CONDITIONALLY COUNTERS A SPELL, BUT ONLY WHEN CAST BY AN OPPONENT.
ABILITIES THAT TRIGGER AT THE SAME POINT IN EACH PLAYERS TURN AND DO SOMETHING TO “THAT PLAYER” (E.G. HOWLING MINE) ARE NEVER OPTIONAL.
THIS LIST IS COMPREHENSIVE. AN ABILITY THAT DOES NOT FIT ALL OF THE CRITERIA ABOVE IS NOT OPTIONAL, EVEN IF IT IS TO THE BENEFIT OF THE PLAYER CONTROLLING THE ABILITY. SIMILARLY, AN OPTIONAL ABILITY IS ALWAYS OPTIONAL, EVEN IF IT WOULD BE TO THE DETRIMENT OF THE PLAYER FOR IT TO HAPPEN.
