Well, the real YSK is that memory and expansion cards have distinctive positions they should take within each slot, with a detente that holds them in place. Your system will only work reliably if the devices are fully seated.
When you first assemble the system, plug and unplug each item several times so you get the feel of it. There will always be a distinct detente when the device is fully seated. It's a lot easier to do this exercise with everything out on the bench, rather than mounted in the case when it will be a stone cold bee-atch to reach in and reseat the parts.