I have to disagree. When you hold the whole piece, you can just put everything in the top housing and just close it from the top with bottom housing. Contact leaf is usually locked-in in the bottom housing with soldering tin remnants, but obviously as from the post above that is not always the case

I don't think it ever happened to me. It also helps if you get an upstroke click and you need to disassemble and assemble it again, It's much quicker to do it with a switch in hand.
Now, when I have switches in the board, I put sliderin the top housing and not on the spring, so every now and then I get a misaligned spring and I have to open the switch and do it all over again. I know the more common method is to put the slider on the spring, and click/tactile leaf around it, but sooner or later, by doing that, I manage to deform the click/tactile leaf by crushing it with top housing.
My $.02.