Any part of the surface that is not smooth will show after you paint it. So if you want a smooth look, yes you need to sand and feather.
Additionally, it is best to use a scraper to make sure you get all of the peeling paint. Otherwise your new coat will flake off prematurely. A pressure washer will just not get all of the flaking paint.
It is a pain, but your paint job will hold up much longer if you really give the old paint a good scraping. You will be surprised how much more paint comes off with the scraper.