Neon is best used for areas that require pops of colour. For example, on this spaceship built by @BloodSpring.
Link to thread
This is, of course, for exteriors. I haven’t done much spaceship/futuristic building myself but I checked through my builds as I know I have done some. Here is a picture of what I made as a normal corridor within my spaceship.
On the whole, I have limited my neon usage to ceiling lights. These lights can go red when the security of my spaceship is breached. Overall, I aspire for realism (yes, even in the case of a spaceship) so I try to follow the normative procedures of lights, which is that they are used to instruct, light areas up, etc. They aren’t really cosmetic at all. I find that in some futuristic games, lights have been added to the most bizarre objects – dumpsters, houses, signs – and it can look ridiculous.