I use a number of different length rods. Most common rods for me are 8 1/2' and 8' 8" 4 and 3 wt's because they fit the type of stream I spend most of my trout fishing on in the Midwest. I have an 8' rod that I thought I'd use more for dry fly fishing, but I lean on the aforementioned more.

I use the longest rod I can get away with for mending purposes, and if I can use a 9' rod for my trout fishing, I do, in 4 and 5 wt's. I have a 6 wt. but I've yet to use it.

For Steelhead and Salmon on Great Lakes tributary fishing, I use 10' 7 and 8 wt. rods. I like the extra foot for mending long belly steelhead taper lines on big water with my 8 wt. and I like the extra foot on the 7 wt. not only for mending, but for high stick nypmhing for Steelehad. I also like the length to help protect the lighter tippet I sometimes use in low/clear water for Steelhead.

Cheers,
Mike.