Being higher, at fixed IMN, will reduce sigma, which will increase the difference between TAS and EAS. In the fixed temperature region of the stratosphere then, the higher you are, whilst TAS will stay fixed, EAS will go down, reducing aerodynamic loads on the aircraft, and potentially drag and thus fuel burn required.
So the higher the better, so long as it stays within the lower stratosphere, and there may be some value in creeping into the middle stratosphere a bit.
I think, it's late, might come back and check my maths in the morning.
G