Nissan specify, in their workshop manual, that the rubber trim, rubber dam and plastic fastener should all be replaced whenever the windscreen is. 
  
I had my windscreen replaced last year and showed them the workshop manual and insisted they replace the parts. 
They agreed to pay for and change the parts and I asked to see the new parts before fitting to confirm. 
  
You can find the workshop manual here: http://www.allcarmanuals.com/factory-service-manual-61-Nissan-350Z-Fairlady-Z-Z33.html 
You'll want page 11 in section "GW". 
  
As a side note - make sure you take pictures before they touch your car as they managed to put a nice dent in my roof when they were fitting the new windscreen with a mallet....  
Took me over 6 months to get them to pay for the repair! 
  
Hope this helps you!