There's more than one way to skin a cat, people. The bottom line regarding scale is that it's whatever makes you happy. End of sentence, next paragraph.
BUT since the question was asked, let me throw this into the mix: Early aircraft, WW-I, biplanes and between the war can be modeled in either 1/48 or 1/32 and either looks great. To me, 1/72 for those types requires an OptiVisor and occasionally a microscope to be able to see the detail. Beyond that, have you tried rigging a Bristol Boxkite in 1/72?
From WW-II on, it's basically a question of your preference and available space. There's also the matter of practicality. A 1/32 B-36, for example, would have an 86 1/4" wingspan. Build an Airbus A380 to the same scale and you'll need to build a garage to house it. As a result, consider different scales for different aircraft types.
Armor is probably best modeled in 1/35, in large part because of the extensive selection. Railroad modelers, of course, have a slew of 1/87 scale armor available.
As for science fiction, I don't know that it's possible to have a standard scale. For example, consider the size of a Babylon 5 habitat in even 1/72 scale. The 'real' thing is 5 miles long (I think), which would make a 1/72 scale model nearly 31 feet in length. That pretty much eliminates the idea of constant scale where science fiction is concerned.
As I said in the beginning, the bottom line is that it's your choice. Besides, every last one of us will build something that's outside of our preferred if it's a subject we've been lusting over and it's the only kit made.
That's my two cents for what it's worth. Or considering the length of this post, two dollars worth.