Did mine today and was really glad. Only 70,000 miles, but the old fluid was brown; new was clear. There are many discussions on these forums about what fluid to use so I went to the dealer. They sold me a qt of Mag1 OEM PS fluid for $6.50. The local dealer has a good reputation and they use this fluid in all Toyota models. It's a very quick, easy and inexpensive job. I'll flush again in 3 yrs or 50,000 miles, or sooner if the fluid looks dark or dirty.
I don't know if it's 'necessary' at 142k but it sure seems like a good idea. Just look at your fluid - if it's not transparent (clear), I'd change it. I don't think there's a consistent color, but it should not be cloudy or opaque.