Hi & congrats on your upcoming litter. Sounds to me like she could go soon being as her due date is tomorrow. Having the experiance of whelping German Shepards I'm sure you know the trouble signs to watch for. You've got phone numbers handy, I assume any necessary whelping supplies, so I'm sure you will do fine.
I've only had one yorkie litter myself so far, so I'm no expert, but I personally will definitely never go without an xray. They're not absolutely necessary, but it makes
me feel better having a good idea of how many to expect so that I know when she is done whelping. And yes, they are also a good tool to show whether a pup is to big to pass or in a bad position to pass. There's always the chance of over or under counting, but my first xray was right on the nose. I'd guess it'd be harder to count heads and/or spines in a larger litter like with your German Shepards.
IDK, don't larger breeds, or all dog breeds for that matter have the same signs of an impending whelp? Maybe not, but I would think so. Temp drop (that stays low for 24 hours), loss of appetite, looking for a quite place or spending more time in her whelping box....or whereever SHE chooses to want to have the pups. My girl chose to have her pups in a corner in our basement; we had her whelping box upstairs in our bedroom.
Good luck to you. I bet she'll do fine and so will you. Be sure to keep us posted and we would love to see pics of the pups AND mom. I'll be checking back for updates.