I usually install my cpu and heatsink outside of the case on top of the anti-static bag the mobo comes with, mostly because my last three heatsinks that i have bought werent capable of being installed with the mobo inside of the case. With the ram i have no preference it is easy to install either way.
I install the CPU, HSF, and RAM while the board is outside the case. I place it on the foam from the motherboard box. I put the foam on top of the motherboard box for a little cushion while installing components.
I install the CPU, HSF, and RAM while the board is outside the case.
I do this too, for a number of reasons. One big one is that fairly frequently, the HSF pushpins are a POS and I would rather just fasten it with nylon screws and nuts. And, if the mobo is in the case, it's like a "floor" that won't give if I slip and bang my hand into it, so it just seems to me like the potential for putting a scratch or a chip in the board is higher.
Also, I've got pretty big hands, so I don't like banging around in the case if I can help it, and often times it can be a tight fit with the RAM if it's close to the heatsink or other components. So I prefer not to mess with that either if I have the choice.
The ram goes in first, primarily because a large cpu cooler may make ram harder to install later.
The primary reason is to insure that the cooler goes on properly. With pushpins, you need to look at the back of the motherboard to insure that all the pins are through and locked. With a backplate mount, you have no option. This is a moot point if you have one of the newer cases with an access hole below the motherboard.
I also test the basic capability to post outside of the case.
I just put the parts on top of the motherboard box, and on top of the antistatic bag.