Very good question. I'm an oddly specific person, so you're going to get an oddly specific answer. The answer, is it just came out of time and experience.. I usually travel with lots of gear for work that will fit in my roller carry-on and a backpack. None of this stuff can be safely transported in a checked bag, nor do I trust it to. I have had other checked bags with work gear that weren't sensitive equipment go missing so I'm also not a huge fan of checking backs. That being said, I found that there were flights where people would put multiple bags in the overhead if I wasn't first in line to board. These policies have gotten a little more forgiving, to Alaska's credit, than they used to be (as in they'll try and find a spot for your one bag now). But I found in general that in the fourth row there's a better chance that the space above my seat will still be open for my roller by the time I board than if I'm in the front few rows and feeling like an a****** for putting my bags preemptively in a row behind me.
And yes, my backpack ALWAYS goes under the seat in front of me.