Soldering is usually required when you are working on a multi-metal piece, such as an 18K and platinum combination. The metal with the lowest melting point dictates the method used. When installing minor components to Platinum, such as findings, soldering is preferred. Whenever you are sizing a ring and need to use solder, use a 1700 seamless solder to do the task. Lower temperature solders tend to show a seam.
Sizing a platinum ring is normally done with the welding method, as long as there are no stones nearby and the ring is made of platinum.
The exception here is Pt/Co, as cobalt oxidizes. To size a Pt/Co ring, the 1700 seamless solder is recommended.