Yeah, if you want to go to the trouble to CFD it, you can, but for the purposes of our hobby, the output of Open Rocket or similar tends to be good enough. In industry, CFD and mechanical CAD are really in two different domains so it makes since that they're separated out like that. I studied ME in college and never touched CFD, people who majored in aero barely/never touched CAD.
But doing full CAD models of rocket builds has so much utility beyond what you'd get from Open Rocket and the like. I'm currently in the early planning stages of a scale Delta II with nine functional strap-ons (a nice, simple means of returning to the hobby
), and Inventor's been absolutely essential in planning out all the intricate structures and mechanisms involved. The fact that I then have all the parts ready to go for laser cutting without extra work is a sweet bonus on top.