Re: How important is rod length?
A 9'6" or 10' rod does offer substantial line control and roll casting benefits over a 9' rod, though nothing compared to a 13'-16' spey rod. 9' and shorter rods however tend to work better for throwing tighter loops and more accurate presentations, thus their popularity in saltwater flats fishing.
The main downside to a longer rod is that it gives the fish a longer lever to use against you. There's a reason why you rarely see the 14wt rods intended for offshore use in lengths over 8'6". (the layout of a handle on a spey rod makes this dynamic quite different)
If you're taking a single handed rod to Alaska and chasing rainbows, steelhead, and non-king salmon, I'd recommend a 9'6" 8wt. A 13-14' 8wt spey would be ideal for those same species. If you're going to fish for kings I'd recommend a 9' 10wt.