"We've had an extremely mild early winter, like the rest of the East coast, and its likely to continue."
How do you know it's likely to continue? That's the problem with being enthusiastic and having temptation come your way. In Mass., I'd bet not very many people are collecting much of anything. Forecasts here in the Middle Atlantic states are calling for the bottom to drop out of the high temps in the middle of the month.
I don't collect anything here in Va. until mid-March. Winter collection can be particularly stressful on evergreens, since they respire through their foliage in the winter-which can dry them out when they have no roots to draw miosture with. You can minimize that with Wilt-proof spray, but it's still a problem.
Frozen cut roots on newly collected older junipers isn't a good thing. Misting junipers--which is probably going to be part of aftercare in collecting such older specimens--could be out of the question since you'd be turning them into blocks of ice if you don't have a frost free place to keep them until winter passes.
Don't make the assumption that the winter is going to be mild all the way through. There is probably still some pretty cold weather ahead.
If you're left with no choice but to collect them now, the point about when to dig them is kind of beside the point
