I agree with one of the responders that time-outs are not supposed to be a punishment but a tool. They were devised, I believe, as an alternative to punishment and spanking. If the behavior you're hoping to affect with a time out does change for the better after he says yes to a time out, then great. It works for him and you.
I actually lost my cool once recently and was really angry with my boys and my 3 year old very calmly and seriously asked me if I thought I needed a little time to myself (my phrase). I said yes and I took a much-needed time out - I even went to sit on the steps where he usually sits! It was great.
A side note: I know I've always had a tendency to ASK my kids too many questions. I'm now in the habit of making statements. If there is a behavior going on that I want to change, I simply say, "If you keep doing X, you will have a time out." I don't give him a choice.
Peace.
G. D