Because of mechanisms like CAF/CFF I have in mind a paper I read some years ago that quoted some work by Sun Microsystems that suggested via hole wall-wall distances in a design should aim to be minimum 0.5mm wherever possible and with the holes staggered as much as possible. 

In today's HDI PCB world a wall-wall gap of 0.5mm seems like a big ask for the designer. Looking at the IPC-9253 and IPC-9254 CAF test PCBs, these have wall-wall spacings that vary between 0.15 and 0.89mm, so I assume the 0.15mm end is intended to represent real-life design and not just a practical PCB fabrication limit?

Is there an IPC PCB design guideline which points to what a designer should aim for as a minimum gap to make the design as robust as it can be?