SF,
I ran into the same question and did a little experiment on my DA 150. In one of the muffler downspouts I did the coil/pre heat thing and left the other one open and just tapped the muffler up close to the engine. In flight, after the engine got warmed up, I noticed NO appreciable difference in the amount or quality of the smoke that each side of the muffler produced.

Hope this helps,
paul