Hey there, Igor.
Having in mind they are all SATA II ports (I'm saying this, because if there were any SATA III ports, I'd have recommended them for the SSD), it doesn't really matter. You can always change the boot order if you ever need to do that via your BIOS/UEFI. So I guess it's totally up to you. Since this is a pre-build system, if you're adding just the SSD, I'd suggest that you connected it to one of the free ports without changing ports for the other drives and CD-ROM.
Hope that helps.
Boogieman_WD