Thanks for all of your replies!
Here's my specific quandry.......I want to knit this fabulous hat
! It calls for Debbie Bliss Cashmerino aran, and size 6 needles. I have this beautiful KnitPicks Andean Treasure, which is listed as sport weight and recommends size 3-5 needles. However, when I knit a gauge swatch on size 6 needles, it matches perfectly. So can I assume that the pattern will be fine using this yarn??