Some support for the can is always a good idea and if using a solid strap, make sure you attach it to the engine mount itself instead of a place on the airframe. Otherwise, you will have a situation where the difference between the vibrations of the engines and the airframe actually increases the load on the muffler.
Many muffler manufacturers leave out any attachment points on their mufflers. In those cases, a simple but effective method is to use a spring (curtain spring or similar), wrap it around the outboard end of the muffler can and attach to the engine box.