You can upgrade directly if you want, no need to install Yosemite.
If you're doing a fresh install it wouldn't matter if you were running Leopard since you're wiping it all out anyway. Best way I've found to do it is with a USB installer but I do this quite often where I work. You can always use internet recovery if it's just one machine and you've got a fast internet connection. I still prefer using DiskMakerX though.