Is it fine using this octane rating?

I have a 73 Plymouth duster with a 360ci engine. The engine was modified for street/strip. It has 9.3:1 pistons, but the weird thing is that even under heavy acceleration, I don't hear any knocking and I'm using the lowest octane (87). I can list some of the main parts if needed, but should I stick to the low octane, or bump it up for the heck of it?