as above they will skim it until its flat. normaly this is only a few thou *(ie 0.001")
10" is = to .254mm and would give a small increase in compression but mainly gain torque and responce,
20" = .5mm or there abouts higher increase in compression (still not much) more torque and response again. but required higher octane fuel or a reduction in ignition timing depending on your current tune/set up.
there are gains to be had but you need to speak with your engine builder and once built advise your tuner of such changes to the tune can be made to suit.
if just replacing stock head gasket have it planned flat and that will be sufficent, if its a performance build then i would be considering the above options among other factors.
cheers
Ryan