Re: How important is rod length?
My favorite nymph rod is a 5wt 9'6" and I find it a very good length in the particular rod I selected. You need to use some caution with 8wt and up weights when you start going over 9'. If you ever needed to cast a rod before buying it is for sure with longer one handed rods in heavier weights. Some of the cheaper rods that are a little heavy in a 9' may be terrible at the longer lengths. In Alaska I was using Sage 9' 8wts and decided to get a 9'6" 8wt. I found the extra 6" in the exact same rod made the tip too soft. I used it a lot but never did get use to the soft tip.
I don't know about you but I dislike tip heavy rods and wouldn't buy a 9wt or 10' rod in anything over 9' with out casting it for sure. In fact a 8'6" length is a good length for a 10wt. As someone mentioned, you have to take the fatigue element serious. I don't think it is a good choice to buy a one handed 10' 8wt and then find it wears you out.
Fishing in Alaska I never thought I had a problem with a 9' 8wt rod. The majority of the Alaska rivers that I fished were not large. There are some big rivers, the Kenai, the Yukon, the Nushagak and the Kuskokwian. These rivers are so large that you need a boat to fish them and a Spey rod is not going to make that much difference. Your best approach is to pick your target water and then pick a rod, or rods, that best meets those requirements.