Yes, low rpm can exaggerate the effect. If the computer/vacuum advances the timing under throttle but rpm is still low then a cylinder will get spark too early and ignite the fuel when the piston is still rising. That's pre-ignition. Higher octane burns slower, so will reduce this problem at all rpm's. It usually goes away as rpm's rise because the piston gets to the top more quickly, before the fuel burns significantly.
So if you need higher octane to get rid of the pinging, use it. (Someone already said use the lowest that doesn't cause pinging.) Or see if you can't retard the timing somewhat if you'd like to run lower octane.
Errant firing may be called detonation, knocking, preignition, pinging, etc. Really, there are probably better explanations but here are the three causes I know of:
1) the normal spark ignites the fuel, but the mixture burns too fast.
2) deposits in the cylinder heat up (like little glow plugs) and ignite the a/f mixture early. (sometimes happens at multiple points or along with normal spark causing multiple ignition points and flame fronts collide - kaPing!)
3) or the mixture ignites spontaneously from cylinder heat and compression. (Under compression, gasoline changes (ask Mola) and has a much lower ignition temp) This may also happen at multiple points, likely from hot deposits and triggered by rising compression. Basically, if it's happening and changing octane fixes it, you're probably getting it from a combination of factors. They're all bad on the engine. Pistons pit, valves burn, rod bearings die (extra compression), spark plugs come apart, etc.
As I recall the
gasoline faq I posted earlier has tons of info on this from smarter guys than I. Check it out.
3rd-cup-of-coffee-David