Tire Storage - any concerns with flat spots?

[sanderGR]

I would like to free up some space and get a tire rack such as:
http://www.tirerack.com/accessories/detail.jsp?ID=164
or the same HyLoft 1000 Multi-Tire Rack Storage

Is there any concern with creating "flat" tire spots?

[mi04se1]

I stored my tires on a piece of cardboard on cement all summer (winter tires) and winter (summer tires) with no issues.

[Seattle09GT]

Sidewalls are much stiffer than back in the day - flat spots are really a non-issue.