ryder22
To answer your question, yes! You must arch your back to a certain extent to get a maximum or heavy lift. As a former competitive powerlifter, I've used the "arched back" to increase my lifts, make them a little easier (relative term), and help secure a solid benching foundation.
Increasing your bench relies on having a solid foundation (footing), all the way up to your upper back being firmly planted.
You'll notice powerlifters will plant their backs, then pick up their butt from the bench, then place their butt on the bench as close to their upper back as possible, while placing their feet a little further back than normal - hence, an arch is born!
Benching with an arch engages your lower chest, lats, and quads. All of these being engaged will add to the poundages that you can bench.
Now, arching does NOT mean lifting your butt off the bench - this would be an incorrect technique. To summarize, arch as much as you can, without lifting your butt off the bench - almost like you are trying to marry up your buttocks to your upper back! And, do this safely without stressing or overstressing yourself.
If all you are doing is working out as a casual lifter, not being concerned with heavy benches, then your natural arch will be fine, as aaron_f said. However, if you want to compete and lift heavy - arch!
Remember, it's more about the solid base foundation, with a little arch, engaging your legs, lats, and lower chest. Happy lifting!