You should only lower your purchase price if the property is overpriced. Have your Realtor do a Comparative Market Analysis (CMA) on the property using SOLD comps within a 1 mile radius of the property (the closer, the better) that have SOLD within the last 3 months. This will give you current market price. This is what you should base your offer on - not on list price or type of financing used.
Many sellers will consider accepting an all cash offer that was slightly under asking price so that they will not have to worry about the possibility of financing not going through. How much they will consider would depend on the market conditions in the area such as comparable sales prices and days on the market.