11th October 2012
It looks like the actual size can vary based on the specific material.
Plywood and MDF
For plywood thickness, this chart suggests 1/2" nominal is actually 31/64 = 12.3 mm thick. Nominal Wood Thicknesses - NewWoodworker.com LLC
I found another chart that gives an actual thickness of 15/32" = 11.9mm thick (again, for plywood).
For medium density fibreboard (MDF), the nominal thickness is usually the actual thickness (1/2" = 12.7 mm). However, I've read that it may vary between 15/32" (11.9 mm) and 1/2" (12.7 mm).
For 1/2" softwood lumber, this chart gives a minimum dressed thichness of 7/16" (dry) to 15/32" (green) = 11.11 mm to 11.9 mm.
Softwood Lumber Standard
For hardwood lumber dimensions, you're right: Wikipedia gives 1/2" hardwood lumber an actual thickness of 9.525 mm. Lumber - Wikipedia, the free encyclopedia
To estimate performance, round your material thickness to the nearest simulated thickness: 10 mm ('metric') or 12 mm ('imperial')
If hardwood is the most appropriate material to build these, then you're right that 9.525 may be a more typical dimension. But, when picking a size to simulate, I noted that I can't simulate 9.5 mm material using Reflex... I have to round it up to 10 mm (giving it identical simulation results to the metric version).
Reflex only simulates designs within a precision of 1 mm. (I should have mentioned that the imperial design I simulated is actual 12 mm thick.)
Depending on the material you use, the 10 mm ('metric') or 12 mm ('imperial') simulation results will apply to you.
Does that make sense?