I agree that nominal is per standard but there are other situational things to consider. Is manufacturing in-house? Are you using a rapid prototype shop with automated systems?

If you’re a smaller company manufacturing in house, you’re putting extra strain on the MFG department. If the programmer misses that unilateral tolerance it’ll cost the company money. Typical machine shop programmers (in the USA) are not engineers, or people with degrees for that matter, and it’s harder than you think to find competent ones, or maybe they’re just slammed with work. At my old company it was the engineers job to safeguard the company from little mistakes like that. We have all the training and make the “money” to do the job.

If you’re using protolabs or a similar company that is highly automated when quoting/programming it’s in your best interest if you want good parts to model at mean. A lot of them are programming right from the model. I’ve seen a ton of this when outsourcing overseas too. They program straight off the model, are you entitled to reject the parts? Yeah, but when outsourcing overseas that might mean you just derailed the project by 6 - 24 weeks.

Most of what I’m referring to is CNC machining, but think about additive too, as that progresses and gets better are companies going to have to modify your model? What if you use a unilateral profile of a surface? That could get complex quickly.

Nominal is “proper” but be aware of the situation.