Only you can assess whether these raw clips have value to you. I tend to keep everything. External hard drives are inexpensive - about 10 to 15 cents a gigabyte.
Also, it is helpful if you understand the architecture of iMovie.
Raw clips go in Events.
The Project file is simply a text file in hexadecimal that contains instructions for creating the movie. It will say things like "Go get this clip and start here and end here. Then add this title here and that transition there."
In the case of your clips in iPhoto, the physical clip stays in iPhoto. It is not duplicated in iMovie. When you make a project, the project file points to iPHoto to find the clips to make the movie.
So, if you want to save space by deleting the raw clips - do it in iPhoto.
Once you share your movie to iTunes, you may notice that the project file gets bigger. This is because a copy of the movie you shared to iTunes is kept in the Project file. This is called the Media Browser copy. If you want to send your movie to GarageBand, iTunes, iDVD, Facebook, YouTube, etc. it will send a copy of the Media Browser copy so you do not have to render the movie again. As long as you have the original clips and the original project, you could delete these media browser copies to save space, and you could always recreate them. You can find these copies by finding your Project in the Finder. Right click on it and select "Show Package Contents". The movies will be in the Movies folder within the package. You can delete these if you really are short on space.
I do usually keep my movies in iTunes, but I have started storing them on an external drive. Then I go to iTunes Preferences and set it so that stuff I drag into iTunes is not automatically added to the iTunes Library. I can watch it in iTunes, but it is physically in a folder where I can see it. This works for me with my iMac. If you have a laptop, you might prefer to just keep them in iTunes.
Let me know if you have any more questions.
I stumbled onto this question and answer, and it is really helpful for me. Thanks.
One further clarifying question: I keep all my home videos in iPhoto, and don't really need to export them to iTunes. I love that all my pictures and videos are all together, organized chronologically. Sometimes, though, the videos are pretty long and I want to trim them down. I'm not usually doing any major editing, just clipping out some of the monotonous stuff. I know that I can edit those videos easily in iMovie, but what happens when i do so? Does it modify the original video in iPhoto, thus making the file smaller, or does it create a separate file, in which case I've now unnecesarily duplicated my video.
Basically, I'd love to keep all my photos and videos together in iPhoto, and am trying to figure out the simplest way to trim video length when needed, and to prevent the rejected sections of video from taking up my hard drive space.