does the 24 Vdc supplied circuits with metal chassis needs to be grounded to earth?
Depend on why you want to use the ground.
The main use of the ground connector is for safety and usually is connect to the metal chassis if there is a risk that the main power can  accidentally touch the metal chassis.
Usually if the main power arrive directly to the metal chassis, the use of the gorund is reccomanded (and usually mandatory for normative); if the power of your circuit is galvanically isolated from your circuit yuo could avoid to connect the the ground.

However some times, the ground connector is used also for guaranteer electromagnetic compatibility but this is a different and more complicated argument.

I suggest you to understand very well how work the ground connector, to inform about power supply that you want to use and than choose the best solution: the safety in these cases is fundamental.

I suggest you to read:
Is 20 watts of electricity dangerous? and How much voltage is &quot;dangerous&quot;?