I would think when the freezing temperatures come to stay. Because the cold will affect the roots. You could bury the pots into the ground to winter them over. Just cover them.

Just wait it out. I don't do any winter protection now until after our Thanksgiving in November. Last year it was even later, like the second week of December, because we had a warm fall. You want to be sure that it's going to stay cold before you put them in the garage.

Depending on your Zone 6, it might not be until January that sustained cold weather arrives--at least that is true in my Zone 6.

Kate

I just brought my potted roses into the garage.
I bring them in when the temperature goes below -4 or -5 C. Snow doesn't matter - it's the temperature at night that makes the difference.
I water them really, really well. Then I don't water them until Jan. or so - and then only a cup a month.
I take all of the leaves off. Don't really know if that is necessary - but I like the look of the bare canes, and I theorize that the leaves can't give any stored energy to the plant.
Hope that helps. I've been overwintering my roses in the garage (50 of them) for 4 years with no losses. Any questions, I'd be glad to help. :)
Carol