If I use a blank sql worksheet, sqldeveloper tries to save it seems like every minute. shows the save dialog box. If the worksheet is something I have named and saved, don't have that problem. I use blank worksheets all the time for testing, etc.
That isn't normal behaviour. There are 'Prompt for Save file on close' option and and 'Close all worksheets on disconnect' options in Tools | Preferences | Worksheet, but they couldn't cause this.
Can you post your OS Version, SQL Developer Version, JDK version?
Do you have any add-ons installed?
Are you using version control?
Also see this link for instruction on how to gather more info.