The whole purpose of time out is to give the child forced reflection/composure time. What better place when you think about it but to go lay down and relax and get ready to play again - properly. For many kids being over tired is why they were unable to cope in the first place.

Time out on a chair happened when sending kids to their room became a reward cause they had tv, radio, nintendo, etc. and more toys than the local store.

I tend to use the couch for time outs when used with the idea of it is a comfortable relaxing place and the ideal environment for regaining control and/or the child lays down and rests. Length of time depends on when child is settled and returns to play when they are ready. If I have to send them back to the couch to try again then I decide when they are ready to play again.