Once again you always do when working with measured quantities.
Quote from:
http://www.statemaster.com/encyclopedia/Significant-digitsNote that exact numbers obtained by counting discrete objects are not subject to the rules of significant figures and should be expressed as exact integers.
This is just a lazy way to deal with significant figures.
A natural number has infinite number of significant figures.
For example 17 = 16.99999999999999......................
Back to the original question.
17 has infinite number of s.f.
117 has infinite number of s.f.
Hence 17 x 117 = 1989 has infinite number of s.f.
So there is no need to make any exceptions. You always do when working with measured quantities.