I do a lot of cutting and etching, from wood to acrylic. About the only time I don't use the air assist is when I am etching on Rowmark Material. The air assist deposits the etched debris back on the hot material where it sticks. However when cutting oak, cedar and acrylic I always use the air assist. In certain spots on the wood there might be more fresh "sap" which flames up. and on the acrylic, every once and a while it will craze the acrylic.
I am using an Epilog Legend 32EX 40 watt.

Ray