Are you talking about mixing the ferric oxalate and the metal salt together just prior to coating? If so, room temperature is fine, unless you are trying to use a super saturated metal salt such as potassium chloropalladite or cesium choloropalladite. Both of these metals will need to be warmed to about 100 degrees to completely go into solution. i don't know a lot of people who use either one for precisely that reason. (Although cesium chlorpalladate gives a unique straw golden color to a DOP palladium prints that can be quite arresting and unusual. Double coating is necessary to prevent solarization, however)

If you are talking about mixing the constituent solutions from powder form, then yes, heating the distilled water to about 120 degrees will help get the powdered FO and metal salts into solution.