If you have a 15Khz screen, then you should use ATOM15.
Using ATOM15 will help to protect your screen from receiving signals that it cannot display. This is useful while Windows loads and until the CRT_Emu driver takes control.
If you have a multisync (15/14/31Khz) monitor then you don't really need it, as your screen will be able to deal with the BIOS and Windows boot screens prior to the CRT_Emu driver taking control.