I had added base rock to my established tank when I had it going and never had a cycle (granted it was not a lot of base rock...) [/qoute]
Thats because the base rock doesnt "cycle" like other live rock does. The only reason you'll see amonia spikes with live rock is if the bacteria has had die off and cant fully break down the amonia yet. With dry base rock, there's no bacteria that has died off for one, and for another, you've already got the established live rock currently cycling the tank.
Sure base rock might have old dead organics in it at first, and they might cause a spike, but its usually just going to be nitrates and normally isnt amonia. Its never been amonia for me anyways and i've added base rock to several tanks.
One way to get the base rock from leaching dead organics or whatever is to clean it really well and boil it if you can. Its already dry/dead, so its not like you're gonna kill anything. If its boiled that will get rid of a lot of stuff (and can also help with future algae caused by the rock)
I've even heard of people using a mild bleach solution on base rock to do this.