RE: DX 8 and a Knife Edge Mix
You can turn trims off (Trim Include) on each and every mix. It depends on what you're doing if you want it enabled.
You might also want to think about using Flight Mode Trims (Trim Setup screen). This will give you a different set of trims for each flight mode. If you use the FMODE switch for your KE mix, you would have independent trims during KE vs. normal flying.
Andy