I have the CPU/cooler/GPU/RAM installed in my Mobo, but it's not in my case because I'm waiting for a monitor to test boot(day or so), is it safe to leave the board on the wooden table i put it together on, without any protection, until my monitor arrives.
Yes it should be safe, just make sure that no dust gets into your components,
I would recommend putting it up or covering it in someway just so that dirt and stuff doesn't get into the PC that could be bad. also maybe just place it into the case and put the lid on the case so that nothing can happen to the MOBO or any of the pieces.
You don't have to put the screws in but just place it in their and cover it so that it stays clean and nice