I ALWAYS select 6F whenever possible, even when 6D is already occupied. If 6E is also occupied then I might consider another seat. I've never been in 17 cuz I don't like being stuck in the middle of the plane. My priority is 6F-6A-7F-7A-8F-8A-9F-9A (then followed by D and C seats).
Also this might sound weird, but I never put any stuff under the seat in front of me. The floor is nasty. I always go for the overhead bins.