A #6 rod should be ok with any salmon/trout/ steelhead up into the 12 to 15 lb range. Particularly these days as rods tend to be stiffer and have more power in the lower half of the rod than they did 20 years ago. When I say ok I mean you'll have a reasonable chance of controlling and landing the fish That's my experience in any event. As other people have said that comes with lots of caveats like how much experience you have with larger fish, the water you are fishing (don't fish Thompson Steelhead with 5wt ...please), size of the fly, the line (using heavy tips etc). If you want to fish salmon or steelhead regularly buy an 8 wt or maybe a #7. If you are trout fishing in still water with the odd trip to river or stream, maybe give pinks or coho a shot then a #5 or #6 will be ok.