And Hercules is safe for Nitro finishes of Guilds, which I at least would always question for the others.
http://www.herculesstands.com/faq.html#
"Is SFF (Special Formulated Foam) safe for Nitrocellulose lacquer finish?
As a professional stand manufacturer, HERCULES goes to great lengths to provide the highest quality and the most secure stands. HERCULES had conducted numerous experiments on our SFF foam and is pleased to find that it does not react with most of the guitar finishes in the market, including Nitrocellulose. The tests included subjecting the instruments on the stand to heat, cold and normal temperatures for periods of up to six months without movement.
However, each manufacturer has their own formulation of Nitrocellulose, with differing proportions of the chemicals involved. There are literally thousands of different variations of lacquer in use today, making it impossible to test them all. Therefore, to provide your valuable guitars with the best support, we recommend you to refer to your instrument's owners manual for more information regarding the proper storage of your instrument."
EDIT: Cooperstand also seems to be o.k. for Nitro: "Cooperstand does not recommend the use of commercially manufactured stands for long term use, permanent display or storage.
Our products are tested safe for finishes we have tested, and most instrument finishes on the market including nitrocellulose lacquer."
Ralf