hmm oki I did some more digging:

http://www.wowwiki.com/Making_a_macro states:

Code:
/castsequence reset=10/shift Spell 1, Other Spell, Some Item

reset=n/target/combat/shift/alt/ctrl
Where n is the number of seconds of inactivity after which the macro should be reset. In other words, if more than n seconds pass without the macro being called, then the next time you call it the sequence will start from the first spell. Note that this is not the time since the first spell in the sequence was cast, but rather the time since the macro was last called (to cast any of the spells in the sequence). This is a very important distinction because it means you cannot use a reset timer to account for cooldown.
This means that if you spam this castsequence

Code:
/castsequence reset=15 Immolate,Conflagrate
in it will cast every second time you activate the macro Immolate. After 15 seconds of inactivity (no macro activation) it will reset to immolate.

Code:
/castsequence reset=15 Immolate,NIL
This will cast it ONCE !!! and then never again (until a reboot probably)

This if you use a NIL or NULL or (whatever) at the end of a castsequence macro be sure to have at least reset=combat or reset=target



(nb: this was probably information which is somewhere else on this forum, but I couldn't find it and I still see many castsequences with cooldown of the spells.)