Yes, but Turkish blocks off seats from self service seat selection (kiosk/online) around 25 hours before a flight is scheduled to depart. Your selection might be limited. This self check-in kiosk trick works mostly for the second flight if you're connecting via IST.

You can change seats up until your boarding pass is issued (it's a separate step from checking in). Don't accidentally press any buttons saying something along the lines of "print my boarding pass/view my boarding pass" after checking in.

Also, the self check-in kiosk let's you change your frequent flyer number, but online check-in does not.