I apologize in advance if this question has already been asked and answered elsewhere, but why do RP-1/LOX rocket systems use helium for propellant tank pressurization? I understand the fact that O2 and N2 have boiling points in close proximity precludes the use of N2 for pressurizing the oxidizer tank, but couldn't the O2 be pressurized autogenously and the fuel tank with N2?
Bottom line: like so many things in rocketry, it's a design trade, usually at the system level.
The book Pressurization Systems for Liquid Rockets goes into great detail on this. I highly suggest you read Chapter 2.1.2 "Selection of System Type" which pretty much covers your exact question. There is even a case description on page 33 where the author goes through the trades for selecting helium versus autogenous pressurization on the S-IVB.
Here's one of the charts from this book describing some of the trade space where weight/compatibility/complexity are considered.
And here is the S-IVB discussion (I am posting it as an image, because whatever format Google Books uses does not allow me to copy the text). Note that the trades were made on weight and reliability.
Edit: The linked document is available at NTRS which is nice because the Google Books format is annoying.