Is it safe to shutdown computer without unmounting NTFS partitions?

Yeah its safe. Ubuntu unmount the NTFS partition during shutdown.You can also set your NTFS partition to auto mount so that you don want to mount it every time you logon.

To auto mount ntfs partitions in ubuntu, you must install ntfs-config

sudo apt-get install ntfs-config

You can also edit your fstab file to auto mount your NTFS partition,but it may be little confusing one for a beginner.See the below post to edit your fstab file to auto mount your NTFS partition but be sure what you are doing is right.

http://ubuntuforums.org/showthread.php?t=785263

The Disk scanning is a natural reaction of modern operating systems (Ubuntu would do it if you had a driver for your ext4 partition on windows too) to other systems writing data to their filesystem. The OS just scans the disk to "fix" "errors" that it might have picked up from the other system's activity. In reality the "errors" usually don't exist, but it's just that the $logfile is marked as if the system needs a check, so of course the software does what it "thinks" is best.

As Karthick pointed out, your system unmounts all drives on shutdown, so no worries.