In my opinion, that's never a bad investment.

Never.

Timing belts are funny things. If you run 50k/year, they'll sometimes last for 150-200k. But if you drive very short trips or let the car sit for winters (for example) the belt might rot before it has achieved the miles that are recommended for service. Always remember that the manufacturer bases it on a safely-achieved mileage. therefore if you sit in city traffic for 70,000 miles, you might have 10x the revolutions of the engine as if you had traveled 70,000 miles on the highway.

Most people have an average use history. Even if it's got low miles, the age would probably indicate replacement.
--
Chris Herbst
1992 745, 68k

And others:
93 944, 150k
90 245, 110k
88 744, 160k
87 245, 185k