Prism99

How much extra you need depends in part on how large the quilt is. An inch extra on each side would likely be sufficient for a lap size quilt or smaller. I would leave two inches on each side for anything larger than that. A lot depends on the level of risk you want to take in terms of running out of space if things shift.

Another thing that affects shifting is the batting you use. Polyester batts are more slippery than cotton or wool and tend to shift more.

Basting method can also affect shifting. I have gotten the least shifting with spray basting; more shifting with pins and thread basting.

Edit: I should add that pinning the edges well will not affect how much the layers shift. If you get to an edge and you run out of batting before you run out of top, it won't matter if the edges are pinned. Pinning the edges simply protects against things such as the backing inadvertently getting folded back while you are quilting.

Edit 2: Actually, running out of batting isn't the worst problem; you can always add a strip of batting to an edge. It's running out of backing or top that creates a bigger issue.