You need to watch the ones that come with the sofa. Unless they are just scatter cushions, if they are big Ones that make up the back then be careful as unless it's very old they are treated with a fire retardant gel due to legal requirements and washing can make it go all bumpy and render it useless. For those I would get an upholstery cleaner in to do the whole sofa.
I had big 24 x 24 cushions on my old sofa and I simply replaced the cushion covers. Freshens up the look too.