If I know in advance that I will spit out what I am putting in my mouth, such as when using mouthwash or gargling saltwater for a sore throat, should I still say the tasmiyah beforehand?

Ustadh AbdulAziz al-Haqqan

Taymiyyah is for drinking, but you can say Bismillah when you are doing an action as well as it is generally seeking aid and barakah from Allah.