When this happened to me, the APU wasn't the biggest issue. Accepting you have to notify the airport authorities who will in turn call the fire brigade, you'll quickly find out that this biggest issue is your passengers sitting over a pool of fuel. In my case, the pool size was five to 10 litres but the fire brigade wanted the passengers off. They didn't care if the APU was running or not but after everybody is off, why would you need it? Also, if the fuel spill was the result of an aircraft fault you'll find you are going nowhere soon. Also, the clean-up crew will do a better job without the unnecessary stressor of APU noise.