Do not use clear plastic. It will get too hot on the nice days and the plants will start to come out of dormancy too early. Then they will freeze back when the temperature dips back down below freezing. If you are going to cover your frame for the winter use white plastic or clear plastic that has been white washed with white latex paint. You must also water during the winter if you are going to cover the plants with plastic.
Dehydration occurs very easily during the winter, especially under plastic.
For the home gardener I recommend not covering the propagation frame for the winter. A covering of light fluffy snow actually protects plants from harsh winter winds. Let Mother Nature take care of your cuttings over the winter. Sometimes she does a fantastic job, and sometimes she reminds us that we are tinkering with nature. This method of rooting hardwood cuttings can and will work for a variety of different evergreen plants, both needled and broadleaf evergreens.