I shared a few forms in more than one project in the past. Wasn't long before I decided it was not a good way to go and made copies of those forms for each project. The ones that were going to stay static I saved as templates and then just added them to future projects as needed.
The two biggest issues I ran into when sharing forms or modules was 1: the location of the file which made it difficult to copy one of those projects to another location as it was easy to missed the shared files. and 2 if I needed to make a change to the code in one of these shared files it could have an adverse or unwanted effect on one or more other projects.

In the end it made more sense to keep each project self contained and not use any code files that were shared and instead use copies of them as needed. Sure you can end up with multiple copies of the same exact file in different locations but that is not such a bad thing.