When visibility is low and water high, like January, I tend to use heavier leaders as they are more forgiving in fighting a steelhead in faster current and also bring in a fish quicker for faster release if needed. (also, less frequent need to check and change leaders for abrasions)
Both steelhead I landed this season were on 15lbs mono used as leader.
I also carry 12 lb leaders, but have not used it much so far this year as water visibility did not warrant it.
Above the swivel, I normally use 12 feet of 20 lbs mono connected by improved albright knot to a 30 lbs braid line.
I use 30 lb braid as main line, not because I need the strength but find them to be easier to work with than a thinner 20 lb braid in cold weather.
I think I also tend to use heavier line than for average steelhead because in the back of my mind I am thinking about being ready to land that once in a life time fish (20+) if I ever hook one.