Personally I find when first creating a project it is best to create a folder to put the project into. I normally create a folder first and then use the startup screen to find and place the project into using the Start a new project option.
Then if the project ever needs deleting, all I have to do is delete the folder.
If you do want to delete projects make sure if you want to keep the original video clips not to delete those. They do not have to reside in the same folder.
If you are using the folder that the video clips reside in then it is probably most safe to open the folder and delete any project files manually so as not to delete the video files by mistake.
Others may have different suggestions on the topic.