I voted warm, but it really depends on the tank and the quantity of water being changed. On my 20 gal, there is enough water volume that a 3-4 gal PWC won't drop the temperature much, so I'll add cold water, but me 3.5 gal tank that a very overstocked quarantine tank gets a 30% change daily, and cold water would drop the tank temperature several degrees, so I use warm water. My 10 gal gets a 20-30% PWC, and I usually use room temperature water...
I think it's a matter of using a temperature that won't shock the fish.