I have heard of kids breaking their boards cause they screwed in the mobo first and then were trying to push the processor/heatsink/memory etc down on it too hard.
I usually install
2) heatsink-fan and/or Memory
I also install the Graphics card or any other PCI cards....leave them in for a couple of hrs and then pull them out. makes it easier later when you're installing the cards. opens up the slots a bit. just my experience.