If you want to backup your original X10 firmware files all you have to do is go to
C:\Program Files\Sony Ericsson\Update Service\db\13740270\blob_fs
The hard drive and 137** number depends on your phone/pc config
So when you open the blob_fs folder you will get a bunch or files, just arrange them by size large to small. When you do you will find 2 large files. 1 around 130 000 - 120 000 kb and another one 16 000 - 12 000 kb. To honest the numbers can vary.
So just save these two files on your hard drive, so next time you need to go back to your original firmware just use these files when your using the omnius method. If your going to use the SEUS method just replace the files in the blob_fs with your saved back up files and continue to flash your phone.