As per Cisco RSTP is backward compatible with STP 802.1D. All the documents in Cisco specify that a RSTP enabled port will go STP when connected to an STP enabled network.
In most cases this is true. In most cases RSTP is backward compatible with STP.
But there is a rider to this generalized statement. RSTP is a lot more faster than STP. The typical convergence time of RSTP is less than <3secs while that of STP is about 50 seconds assuming that no proprietary cisco enhancements are used.
The rider is, RSTP and STP work perfectly together only IF RSTP is used in the core while STP is used in access-layers. Any other combination of RSTP-STP would be a disaster. IF done any other way, a STP loop would be induced into the network.
The above link gives a recommendation.. I quote
"When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances."
I have faced problems personally in RSTP-STP interoperability.
If there is need to convert a network from STP to RSTP, I would suggest that the Core is first converted to RSTP.. then the distribution and then the access. (Although this methodology is not entirely tested).