Default answers:

1: Read the approved flight manual and follow it.
2: Any remaining unanswered questions, ask the factory.

Now on to something else. The articles Papa3 referred to came to a head in the August 1977 Soaring magazine. Homebuilders Hall was written by Stan Hall a Lockheed engineer and the designer of a number of homebuilt gliders. The question "which determines flutter IAS or TAS?" came up and led to a number of contradictory answers which he endeavored to resolve by bringing in Perry Hanson, a NASA aeroelastician who spent much of his career studying flutter. If you're an SSA member you can look up his full answer in that issue of Soaring online. To simplify his answer assuming the the conditions are not such that limiting Mach number is a factor, and assuming the structural characteristics haven't changed due to temperature changes a glider will always flutter at the same Equivalent Airspeed - which is nearly the same as Indicated Airspeed (assuming the airspeed indication system isn't so grossly inaccurate as to diverge significantly from Calibrated Airspeed) except that EAS is usually a little lower than IAS. The example he gives is that 120Mph. IAS at 35,000 ft. is equal to 118Mph. EAS. Essentially he says that if you're worried that the glider will flutter at 140Knots at 35,000 ft. you should be just as worried if you're going 140Knots at 2000ft.

I've found some glider manuals give no reduction in VNE as altitude increases, some do give reduction in VNE as altitude increases. The reduction in VNE I have seen in manuals frequently doesn't seem to be done in a manner which is proportional to changes in TAS. It's left me wondering whether the VNE reduction actually is done with changes in the structural qualities with temperature is the reason behind it. Or whether it's a CYA restriction. I'd love to find out for sure just to put my curiosity to rest.

For myself I always go by the first two answers I gave. As it is, I have no oxygen system in my ship and I fly in Canada where there are damn few places you can go above 12,500 ft. at will anyways so it's all kind of academic to me.