That really depends on what you're making. I've made a lot of things that are basically, I want an object with this shape, model that shape, print that shape, success. For things where tolerances and material properties matter, a little trial and error takes care of a lot of it.

There's an engineering saying that anybody can design a bridge that won't fall down, but it takes an engineer to design a bridge that just barely won't fall down. Why do you want a bridge that just barely won't fall down? Because it's a lot cheaper to build. That's not much of a concern when you're printing little doodads at home. I waste some material by designing overly-strong structures, or getting it wrong and iterating. That's fine, the stuff is cheap.