The key is being able to respond promptly and being committed to the answer one has given which aligns with the point you raise on timeliness. There are two discourtesies:

1) Not providing a committed answer until the last minute; and 2) Reneging on a previous commitment, particularly at the last moment.

Responding promptly is important if only to say, "I cannot commit at the moment but I will let you know for definite by [date]", where date is sufficiently in advance of the event in question.