I wouldn't stop de SQL Server service because than you wouldn't be able to execute any queries against it. You can use the following code to create a backup.
The osql.exe is replaced with "sqlcmd" in SQL Server 2005
The only reason why you should stop the SQL Service is when you just want to make a copy backup using the "COPY" or "XCOPY" command in de command line.
More information on the command: