It is important because of the snow load factor. If you lived in Florida you could get away with a 1" slope, although not advisable, since your only concern would be rain water. Since snow is involved, your minimum slope should be 4 to 1. For your area I would suggest a 6 to 1 slope. Since you are using 2x8's for your rafters the vertical end cuts for a 4" slope is 7 and 5/8's. If you go with the 6" slope then your end cut will be 8 and 1/8th.
The rafters are not as critical as your beam size in relation to your post distance. This is what will actually carry the load. Good Luck!!!!!