Help on purchasing new vehicle

Im planning on purchasing a new vehicle this weekend. Ive shopped around and have found the dealership im planning on buying from. Ive been pre-approved through my bank and have gotten the OTD price. The vehicle im getting has already been marked down twice to a bit over $5,000 since Ive been monitoring it. Should i try to negotiate lower. Or have them add anything extra. Also is it worth having them run my credit to see what rates they offer and if i dont want them to run my credit is it mandatory for them to do so? Any tips are appreciated thanks.