My 16 month old Rottie xGSD's legs click or crack at the back when she often gets up or stretches.
Could this be a sign of something?
She runs and plays happily, does agility and chases her frisbee and has never limped or looked uncomfortable on them.

I click and crack all the time (i am only 19 )- it just means loose ligaments which allows for air to be released when the joint moves. The air releasing is what makes the noise. If you are worried then get it checked out

Did try to get it checked out, but my girl hates the vets and went so tense they couldn't stretch her hind legs an inch, they said only thing they could do is to x-ray her hips.  Didn't really want to do that as she is not limping or seeming in any discomfort.
So just wondered if anyone elses dogs had this problem and if it was caused by anything or gave them any trouble at any time.

my schnauzer is 3 and a half and she regularly clicks when she gets up- no other symptoms though.

One of my labs clicked as a puppy and occasionally now and I worried about hips etc but she is fit, healthy and has a hip score of 6 : 3.

My hound clicks and he has arthritis.  He is 9 years old.  I dont want to alarm you but considering the two breeds that your dog is a cross of I would investigate further.  The way you manage any potential hip problems when your dog is young could make all the difference later on!  I'm assuming that the parents weren't hip scored given her type?
Sarah