Typography of Letters in Circuit Symbols
That depends. Generally, use whatever is clearest, least ambiguous.

However, there's different standards that might apply. For example, if you publish a circuit, it'd be generally good style to follow good ol' DIN standard formats – which include a specification for things like page frame, numbering/naming of schematics and a DIN standard font. That font isn't exactly great to read (IMHO), being optimized for old pen plotters; however, there will never be an ambiguity between let's say a V and a U, an l and an I and a 1, or an O and a 0...

If you're contracting for some larger company, especially in defense, different standards might apply to you. Make yourself familiar with the demands of your audience! Remember: asking early is always better than being sorry after.