I think Almond will be OK. I mentioned unscented because most of the other Dr. Bronner's "flavors" -- Peppermint, Lavender, Eucalyptus, and Citrus (the last one is Rose, I believe) can be used effectively to repel one insect or another. I believe the soapy water suffocates them by blocking the spiracles (breathing holes). Have you seen this video? http://www.butterflyfunfacts.com/monarc ... -video.php
Soapy water is effective for killing almost any bug, including ants and flies. Most certainly, it could kill beneficial insects if they get caught in the spray. Part of my reluctance to spray aphids stems from concern that I might also end up spraying ladybug larvae. I only spray when there's a bad infestation and there are no ladybugs or larvae on or to put on the milkweed.
Soapy water also helps to make the aphids let go -- sometimes the aphids won't come off the plant unless you use water spray that is hard enough to also damage the leaves, but if you spray with soapy water first, then the water, they'll come off like a charm.
I don't know what effect the soap residue would have on the cats if they eat the leaves. I don't know for sure but soapy residue may not allow the eggs to stick properly.