If someone were to accidentally spill something on Tseng's suit, what do you think his first reaction would be?

I suppose that would depend on who did it. If it was Reno? Likely have the overwhelming desire to shoot him on the spot. Although you would never know it, because we know Tseng never showed emotion.

If it was anyone else. Be his calm self. Excuse himself, where I assure you he will have an extra perfectly pressed suit in the back of the car, he will have changed into in a second.

That is taking into consideration he does not use the Shinra product we all know they must have that prevents any such catastrophe's. Wrinkles in suits, hair that is out of place. It is very possible anything falling onto Tseng's suit would roll right off.