okay you connect the end of the static strap to the case, plug the power cord in (but dont turn it on) this gives a wire from ground, onto the metal case, and onto you. Do this and youll have 0 chance of damaging your parts through static.
Secondly, yes its fine to leave the mobo out while installing parts. I do it everytime. Once i have put the CPU, ram and AGP/PCI cards in, i punch out the slots in teh case where the expansions cards are going to go, and screw the mobo in. Its very hard to install things like a heatsink on a mobo INSIDE the case. By the way, be careful when installign the heatsink. Make sure you use a flat head screw driver and hook it in properly before you start pushing. dont wanna end up slipping and cracking you mobo