I'm not sure about a specific percentage, but I would also check other sources for rates if you haven't already.

Hotwire doesn't always have the lowest rate - I checked hotwire for an upcoming 4 day rental in Las Vegas - hotwire was $24.95 day for an economy car plus close to $54 in taxes/fees - over $150 total. On Orbitz I found Dollar and Alamo were both less than $20/day, with taxes/fees closer to $35. I made a reservation directly with Dollar for around $108 total, which I can cancel without penalty if a better deal comes around. Alamo was around $114 total.

If I just took 20% off the hotwire rate, I'd be bidding around $20 on priceline, which with their taxes/fees is already what I'd pay to Alamo directly. So I would definitely check around for prices beyond hotwire.