If they have been washed well, I would think they would be all right for watering a garden. Sometimes we add just a teaspoon of soap per gallon as a surfactant. I do not think this would be any different. Do you know what kind of soap was in them? As long as it was not antibacterial, I believe it should be fine.
One thing to remember is when purchasing any items used for the collection of rainwater, tell the person that you are using it to harvest rainwater. Any items purchased for harvesting rainwater is sales tax free.
http://www.twdb.state.tx.us/publication ... dition.pdf