He said it's because they're all "bull nose" fittings to the radiator,
and olive compression joints are better. I replaced several just
recently with drain cock type valves and I don't remember seeing
anything about them being unsuitable for pressurized systems?

At this stage it is better to replace all rad valves. They are now old.

It won't be necessary if they have been recently replaced. I use drain cock
types and my system is pressurised.

Christian.