The picture of the jassids below stresses an important thing I'm firmly convinced is true when it comes to fly fishing (perhaps especially dry fly fishing). It is much more important to have different sizes of a fly pattern than to have a lot of fly patterns in the same size. That's why I always tie my flies in different sizes. Perhaps not like with I've done with The Jassid (that was more like a challenge) but a reasonable variation in the size of your different fly patterns would most likely help you catch more fish. So varying the size of a certain pattern (according to what insects are around and what the trout is feeding on) rather than changing to another fly pattern all the time is the way to go. At least it has worked well for me.
Have fun with your favorite flies,