I'll push them right to their rated limit for many of my own projects since most reputable manufacturers are already building in a 20% margin.
That said, I'll also subtract 20% or more if the ambient temperature is over 40C, 20% or more if nearby devices or materials don't appreciate the extra heat, 20% or more if the application is professional and intended to be very reliable, 20% or more if subsequent replacement or troubleshooting will be especially difficult, etc. That's easily 1/3 to 1/2 of rated power if multiple factors apply. And don't forget to leave a healthy gap between the resistor and the PCB, maximum rated temps for resistors are at least 100°C hotter than those for PCB's.