The Corolla, or even the Camrys should be able to run on 87 gas without pinging. If the engine pings it usually means it's not as healthy as it should be (e.g., lean mixture condition, excessive carbon buildup in the combustion chamber, plugged fuel filter etc).
You already found the advantage of iridium plugs that it's thin center iridium alloy electrode allows for a lower firing voltage and the ability to ignite leaner mixtures (normal is about 14.7:1, the iridiums can ignite IIRC 17-18:1 lean mixtures). I know, these are great stuff.
The prices have come down a lot for these excellent plugs (~$12 -> ~ $6). For those who still have to save a couple of bucks over NGK Laser Iridiums or Iridium-IX, try Autolite Iridium XP with 5 years unlimited miles warranty. $4.99 after rebate at Schuck's or Kragen. However, it's said to have less Ir than NGK.
Select plugs only by using the application catalogs and not "cross reference charts":
NGK:
Autolite Iridium:
Another thing I've noticed since starting to use these, is that I've not
> only gotten a slight MPG increase ( maybe 2 ) but can now use medium
> octane instead of high octane in the 93 Corolla wagon ( 1.8 EFI ). >
> That won't hurt the injectors or anything will it?
>
> When I bought the car, they said I could alternate between medium and high
> octane tankfuls, but when using medium it used to run a bit rough. >
> Now it's smooth, and I save about 10 cents a gallon.