Hi,
in short:
RD makes unique VPNv4 prefixes. If you have one customer and his routes differ in the IPv4 part (they usually should), then you get unique VPNv4 prefixes with ONE RD for this customer. In this case you can reuse the RD - it is even recommended.
RT defines VPN membership. So on one PE you mark routes with a specific RT and on another PE you insert routes based on this RT. If you have to build an any-to-any VPN for ONE customer, all his routes could be marked with one RT. This RT has then the meaning "Customer X" in your environment. In this case RTs can be reused - indeed once again it is recommended.
If you have a second customer, it should most likely never get to see routes of customer X. So you need a different RT for your second customer and so on.
For a more detailed description you could read f.e. through
http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a00800e977b.html#wp11131
Hope this helps! Please use the rating system.
Regards, Martin