Its probably best to mount it on the wood if the wood is lower than the ceiling tile. If you still get good signal though while it is placed above the wood and you or someone doesn't want to ruin the wood or because of aesthetic, then place it above the wood.
Here is a doc that explains the two different clips and mounting.
http://www.cisco.com/en/US/docs/wireless/access_point/mounting/guide/apmount.html
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-Scott
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