Some of the "rules" (or standards) make sense, but of course you're free to disregard them.

The five wraps of rib may seem arbitrary at first, but it's easier to get even spacing with an odd number of than an even -- you make sure the middle rib is in the middle of the body. Once you've realized that, and notice that three doesn't look like enough and seven really crowds the body, 5 is a good number, if you're tying for sale and want your flies to look consistent.

Until comparatively recently, most dry flies were tied to imitate mayflies, so the one or one and a half is not only somewhat in proportion to the real insect, but it fit in with the belief that the fly should rest on the hackles, the point of the tail, and that the point of the hook should just rest on the surface. This was what Halford determined to be the best way to keep the fly floating as long as possible. We have different ideas today, and indeed, proportions have changed to follow.

The main driving force behind having standard proportions is to allow the commercial tyer to have all flies of the same pattern that he tied look the same. The only way to achieve this is to have a rule. If the rules resulted in flies that didn't catch fish, they would have changed over time.

On traditional patterns, I see no reason to not use the traditional proportions. On patterns that I'm making up to represent a specific insect, I'll adjust to fit the need -- but I'll still end up with a rule of some sort.