Hi J.,
I would suggest nipping it in the bud. You should start time outs whenever you feel they are necessary. You may have to teach them what you mean by a time out, but that's part of learning. Hopefully, when they realize you are not going to put up with their actions they will become a lot less frequent. Be consistent and follow through with what you say you are going to do. The worst thing we do with our children is say if you don't quit something I'm going to whatever and then we never do the whatever it is we said we were going to do. Always follow through. Don't try to fool them by saying something you would never do because it will back fire. Little ones are so smart...sometimes smarter than we are, so try to stay one step ahead of them.
Have consequences for actions that you don't like, so they will know what is going to happen when they do something wrong. Consequences...time outs in the corner, a chair, in bed, or just a designated spot. You may find out they dislike one spot more than the others, which will make time outs a lot more effective. Time outs don't work for all children.
Good luck!
C.