I have read that if your car has more than 60k on it, then you should change them. I'm not sure how valid that is, plus I prefer to stretch my $ a bit further.
I just recently replaced the shocks and springs on my '91 TT. The car has 148,000 miles on it. As far as I know, they were original. I purchased the car with 78,000 miles, but when you drive it everyday it's difficult to tell that the ride quality is deteriorating. I decided not to keep the factory set-up. Instead, I switched to the Tokico Illumina 5 way adjustables with Eibach Pro Springs. The car was only lowered by .5"-.75". The ride is much smoother, and the handling difference is incredible. I installed them myself, but would not recommend it if you don't know your way around a garage. You must be very careful when assembling and dis-assembling struts. Hope this helps.