What water temperature should be used for instant coffee?
You have quite a bit of leeway with Instant Coffee.  You can brew it using water of your desired temperature. I would recommend using water just below boiling point though, around 95-99 degrees Celsius.  You could use 85-95 degrees Celsius water as well; it won't make a big difference in terms of extraction of flavour, and here is why:

How instant coffee is prepared in the factory:


Coffee beans are roasted
They are ground very finely (0.5 mm - 1.1 mm in diameter)
The ground coffee is mixed with water in percolation columns which reaches 155 - 180 degrees Celsius.  This is the extraction process.
This mixture is then concentrated and/or dried using vacuum evaporation, freeze drying, or spray drying methods.


The result from this is what you get in the store.

Conclusion:

All of the very particular factors that one must consider when trying to make great coffee have already been decided for you.  You have no control over roast, grind, time from grind to extraction, extraction time and temperature, or time from extraction to consumption.  All of this been decided for you.

It is up to you.  Brew it at whatever temperature you like to drink your coffee at. 

However, it may be slightly harder to dissolve the coffee and get good flavor at cold temperatures.  Keeping it on the hotter side may be better.  That's just chemistry though.



Sources:


I am a professional barista and coffee connoisseur.
Instant Coffee Wikipedia Article