I would claim that the rule to use the word "take-off" only for take-off clearance is definitely one of the most important rules of radio phraseology. Like already said above, otherwise you should always use word "departure".
This is an excellent example how many rules in aviation are written to the books and manuals by the blood of other fellow pilots and passengers who have lost their lives during the aviation history. Respect those already written rules and you will be sure that they aren't going to use your blood to write new ones.
However, it is weird to notice if take-off word is still misused in some part of the world. Very sad if the lost of 500+ people at Tenerife was not yet enough...
Safe flights for all of you,
Approaching Minimums