When starting a new RSPAN session, do I need to delete the old RSPAN VLANs that were used in previous sessions?
Your VLAN database will need to specify which vlans are designated as rspan vlans. However, there is no need to delete this vlans from the database, but rather you should leave them there for future use.
You will want to keep things clean by deleting the monitor session on the source switch that you tie to a particular rspan vlan when you are done with it, as 1. you will be generating unnecessary traffic on your network and 2. confusing yourself later by mixing traffic with another monitor session that you may create elsewhere that uses the same vlan.
If you need to preserve the rspan session for a while while you collect data, you may want to tie new monitor sessions to a different rspan vlan.