Deadwood is pretty crowded throughout the summer, so if you're planning on spending the night, I don't know that the rally in Sturgis will make much of a difference. If it's just for a few hours during the day/evening, no big deal; the town will be full but so what? Parking is a bear but there's a trolley that runs all over the downtown area.
Don't miss a visit to Lead ("leed") as well as Deadwood.