I did transmission line theory at University, but it was a long time ago. Impedance for a cable is constant, and not affected by length.

The issue is more that problems caused by mismatches increase with longer cables.

I think you are worrying too much – do what you think is right, and worry about problems only when they occur. Example – AES specifies 110 ohms, but most people use mike cables – who knows what the impedance of a mike cable is ?