Mike,
I always say, if you are tying a pattern for a swap, gift or to be mounted and want a particular pattern, use what is called for in tying.
For your own fishing, I always say, experiment. Let the fish be the judge of materials you used. Think outside the box.
I feel very strongly about subbing different colors of hackle & dubbing. I find one will work well for an hour or so and by switching to a different color grouping, my catching picks up.
The same is for hook size. I normally tie a specific size hook on a pattern that has worked on the water where I intend to fish, previously. When tying I always tie a few flies of the normal size, then tie two or three in a size smaller and again one more size smaller. This has saved my sanity many times.
All of the above is a good justfication for tying way more flies than I will need and is fun for me.
Make your tying fun and relaxing. Then go fish.

Denny