IPC advises in the IPC-7711/7721 that especially in lead-free soldering it is better to clean the soldering tip with brass wool / brush (dry cleaning) because this will prevent a temperature shock on your soldering iron tip. However, you can also clean in a wet sponge. But tap water is not recommended because it contains too many minerals (chlorine, lime, salt, etc.) that will negatively affect the life of the soldering iron tip. What IPC recommends is distilled (boiled) water or even better demineralized / deionized water!