When tightening compression joints it always seems a bit kind-of hit
and miss and they seem to need to be VERY tight to not leak - so tight
I worry about damaging the joint or pipe.

I normally put grease on the threads which makes them easier to
tighten but of course doesn't reduce the pressure on the parts..

After fitting a radiator the screw cap had started to crack after it
was leaking a tiny bit and I tightened it some more. So I'm wondering
if I'm doing something wrong, perhaps the pipe should be pulled out a
little bit from the joint before tightening? or anything else?

Also are the silver joints stronger than gold ones?