Shockwave, yes, if you want to view certain dynamic content. If you have a slow connection, and or little hardrive space, I'd only download the Shockwave player if you frequently play Shockwave browser games.
Flash (the latest version being MX [6.0]), on the other hand, is an animation software program used often by graphic artists and/or webdesigners. To view sites that have Flash-enhanced content, you'll need to install the Flash browser plugin (not the same as the animation software). There are lots of sites out there these days that are designed soley in Flash, and you'd be missing out on a lot of you didn't have the browser plugin. I think there is little or no download times. Just go to a Flash-enabled site and choose to install it. That simple.
I don't have any version of Netscape installed on my computer (sorry, just IE6 and Opera6), but I think I remember having the option to install the Flash browser plugin in the Customized Install settings during the IE6 installation. If you don't already have the browser plugin, download it from the Macromedia website and install it.