I have had a insurance certificate asked for in a couple of venues but no I have never had to file a claim, (it is pretty much just piece of mind).
There have been several injuries at events I have done broken arms, legs smashed faces, busted teeth etc. but they are not the responsibility of the DJ. Now if your speaker stack falls over and hurts someone that will be on you, if you have speaker, power and mic cable spread around on the floor and not in a protected area (read blocked off) and someone trips and falls in the general vicinity then that too is going to fall on you and your insurance.
If anything should happen anywhere close to the DJ area then get the names, phone numbers, and addresses of any and all witnesses and have them write down what they observed. Get a statement from the injured party if possible and be sure and write down exactly what you observed as well in cluding date, time, level of sobriety of the injured party etc.