Is the Freeze process necessary after a Propagation process when using Rotobrush?

Rotobrush relies on a pixel-by-pixel analysis of your shot and it does this every time the CTI (current time indicator) moves. If you do not freeze rotobrush the data is not locked and Rotobrush will propagate again the next time you move through the timeline. Your AEP project file size also dramatically increases as rotobrush is calculating. Rotobrush is not something you can just start and walk away from. It must be watched, the process stopped and corrections made every time it loses the edge. If you do have a shot that is longer than just a few seconds that does not require corrections, you could probably create some kind of procedural matte to create the matte and that would be a lot more efficient workflow. 

Ideally, you edit your video first so that you only have to apply effects to the footage that is used in the final project. Rotobrush was never designed to remove the background from an hour-long shot of someone giving a speech. It is designed to help you create a matte for a shot that is a few seconds long. When you have a shot that needs Rotobrush it is always a very good idea to put that shot in a separate comp, then nest the comp in your main composition. If I have a shot that lasts for longer than a few frames I almost always use the Composition/Pre-render menu and replace the Rotobrushed comp everywhere it is used in the Composition. 

Yes, you must freeze Rotobrush before moving on to any other layer or effect in your composition. Any shot that needs to be worked on should be the only shot in the comp. If the Roto takes longer than a few minutes the comp should be rendered. Difficult shots may need to be color corrected first to improve the edge contrast, pre-composed so that the color correction is used, then processed and rendered so that it can be used in the comp as a track matte layer for the original footage. After you render the matte to a format like GoPro Cineform with an Alpha and Straight color (I always use 10-bit color unless a client specifies a different format). you can delete the Rotobrush comp to clean up your project, reduce the file size, and streamline your workflow. 

Rotobrush should always be the next to the last tool you use to remove the background from a shot. Creating a procedural matte using color or luminance channels is the first, rotoscope by hand is the last. About 90% of my Rotobrush work involves a garbage matte animated by hand, some color correction, a Procedural matte to remove part of the background, and then Rotobrush. If a shot that is longer than just a few seconds cut up the shot and do the rotobrush in small sections that can be rendered and then put back together in Premiere Pro or your favorite editing app. Letting a machine propagate for hours is incredibly risky. Cutting up a 5-minute shot into  10-second pieces, running rotobrush on the pieces one at a time, then rendering one at a time will only take a few minutes longer than trying to process the entire thing and it will be a lot safer.