The scenario you described in the 1st post was for a heavier aircraft in the same conditions so the atmospheric properties would not change. If you're talking about extended glides from altitude then the EAS will be the same all the way down, and if it's slow speed compressibilty won't matter so I guess that makes the CAS and most likely IAS constant. TAS of course will vary. Unless you're in a glider it's rather academic though I'd have thought since in a (previously) powered aircraft you'll have bigger things to worry about than correcting for small ASI errors etc. You'll probably gain a better performance increment by having a ball park figure in mind and correcting it for head or tail wind components (add or subtract half the wind respectively) to gain best glide angle over the ground.