What kind of begonias are you interested in? Cuttings can be taken almost year long but best when you have warm conditions. There are not a lot of "hardy" begonias to put in the ground in zone 6. Start with grandis if you want a hardy begonia. Check PDN to see what they recommend for hardy begonias but they are in zone 7 to 8 (close to Raleigh, NC) and do not have a lot to choose from.

Terra Nova Nursery is working on hardy begonias but they are in Oregon. Another guy (John Boggan) is working on crosses for the Washington, DC area. You could see what he recommends. Ozzie Johnson in our club (Atlanta) has some "hardy-ish" begonias he has collected in China that show promise too. Maybe one of these years (or decades) you will see begonias in the landscape as proven winners?

B. grandis may be ok to do by cuttings but most are done by bulbils (much like tiger lilies).