They should defiantly "float"in the rail an stile.i like to add "barrels",little rubber items you put in the groove before the panel goes in to keep the panel from shifting.Lee valley has a couple sizes.
You may also want to consider pre finishing the panel before assembly
Note that with a mdf panel expansion and contraction is not the issue it would be with a solid wood panel
You may also want to consider pre finishing the panel before assembly
Note that with a mdf panel expansion and contraction is not the issue it would be with a solid wood panel