On KLM you have access to the seat maps on some flights but they reserve the right to charge extra you for those seats - in particular the bulkhead/exit row seats.
I had this exact question. I just booked a DL codeshare on KL metal JFK-AMS-TXL...on the JFK-AMS leg I chose seat 10A on 744, which appears as priority seating on the DL.com seatmap. When, exactly, would KL charge me extra for this? At check-in? I don't want to end up being bumped into a middle seat in the back and would rather just pay for it.