Almost any PCB you see have a couple of mounting holes and there are a few things to remember when designing your circuit board. First of all don’t forget to add them even in prototype boards, it is very annoying to find that you have forgotten placing mounting holes and trying to fix your board using silicone glue.
Another thing to consider is the hole size. You must choose a hole width according to the screw that will be used. If the holes will be plated through you must design your holes a bit larger since they will become tighter after plating, if you are not sure talk to your pcb manufacturer.
Also if the diameter of the hole is not enough and the clearance for internal planes are not enough the thread can damage the internal surface and short your internal layers and make your board totally useless.
Using plated or non-plated holes is another important thing to decide. If you will use mounting holes for grounding then using plated holes is a good idea, but you must not trust thin layer of copper plating in and insure your grounding using some copper area (flat washer) and supporting vias.
Measure the screw head and design the copper ring and copper keep-out with this dimensions in mind.
If you have large space, adding some extra mounting holes is also a good practice, I have use them many times for extra support or when using the circuit in another project.