Seems they can be aligned with all sorts of things and nothing at all. The Tumulus de Kercado in Carnac is aligned right on the midwinter sunrise, yet just up the road at Mane Kerioned there are two dolmens aligned directly south and one to the west.
Down in the Ardeche I discovered about 10 dolmens in one valley all aligned on the same notch on the horizon.
Thing is, if they're not aligned with something astronomical or topographical, there's nothing to say that they're not aligned on whoever built them's granny's birthday, or a slighlty less frivoulous but equally impossible to ascertain event.
I think it's always worth looking to see if there's anything there, just as long as one doesn't get hung up and start twisting the data to fit the model...