jon - on my old (custom) car with short knuckles (losi xxx-s), i would only shim bump if i change the roll center as well. a 2* cahnge in castor is only about 0.6mm vertical change, and usually i am between 4* and 5* so that's half. i don't bother with the 0.3mm (.012") change. that's like a brass motor washer!
i can usually feel 1 or 2 mm of bump steer, but 0.3 is asking a little much from me.
with the ec it's more important as the numbers are about 50% larger than the above if you use the long hole on the knuckle. now we're talking 0.5mm/deg or so which is doable. i have yet to do it, though.