I make this decision based on the cost of the lens. I've only purchased it with my 70-200mm 2.8 With the cost of the lens, it made sense to me to have that bit of extra protection.
I don't know how much the warranty is for the D90. The great thing about B&H's warranty is that is does cover it if you drop it, which given one's level of clutziness, could be a good thing.
I might consider it, given your budget and clutziness