Should we be doing preflight calculations of Vne IAS reductions when
planning aerobatics at 9000 ft. or soaring or high speed flight at
15-18,000
ft?

Many gliders capable of Vne around 146 kias below 10,000 MSL actually have
lower IAS Vne charted for altitudes above 10,000ft MSL and in 3000ft blocks.
The DG1000 has the following limits.
Vne at 10,000ft is 146KIAS, 13,000ft is 138KIAS, 16,000ft is 131KIAS,
20,000ft is 114KIAS, 23,000ft is 117KIAS and 26,000ft is 111KIAS

Vne is based on IAS, TAS for the same IAS is higher at higher altitudes. But
remember, it's not just airspeed, or airspeed indicator errors associated
with altitude changes, but also other dynamic "q" factors on the airframe at
higher "effective" airspeeds.

Many jets have a Maximum operating Mach limit. Vmo would not change with
altitude, but the IAS limit associated with that Mach limit will continually
decrease as altitude increases. Eventually that Vmo (IAS) gets down to
Vstall, and that is the "coffin corner" in a previous post.

BT