I just had to re-tram my machine, so I took the opportunity to make a video on my process for tramming the Z axis and the spindle axis.
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Thank you for posting it. Your videos are a great source of information.
I have a similar spoilboard to yours, and when I was tramming my spindle I noticed that putting pressure on the spoilboard was enough to move this dial indicators by a few thou. I tend to think that doesn’t matter too much since I usually do spring passes to finish things, but was curious on your thoughts. Perhaps I should glue two pieces of MDF together like I have seen some people do.
It really just depends on what you are doing with your machine as to if it matters or not.
You are correct, just about any top will bend a few thousandths if you push on it much at all. The aluminum spanners will also easily. However, you can push on the spindle in any direction and get 5-10 thousandths w/o trying to hard as well. This isn’t a high end milling machine for hard metals after all 
My spoilboard is 1" MDF, and I kinda feel like that is the minimum stiffness for a CNC top. It is pretty stiff, but will deflect pretty good when you push down on it. Next time I build a spoil board I will probabaly laminate two 3/4" sheets to get a little more stiffness, and also this will make the dog holes longer so the stops and clamps I use won’t rotate as much when under load.
When you are doing something where you are concerned about deflection, it is best to take care of that in your toolpath by making your laser pass light (especially when using a down or up cut bit) so the vertical loading is minimized.