Installing headers -- While engine pulled or after re-installed?

I haven't installed anything but Doug's since TTI rust issues early 2000.
The last 1/2 dozen, didn't have to ding one of them.
I've learned the engine will shift around quite a bit by loosening ALL the motor mount bolts and getting the engine where it's supposed to be, to help.
I also don't hesitate to use a ratchet strap around the trans or T-bar to shift a tube/collector over an 1/8 inch-ish, enuff to stop a rattle, and Ive been known to stuff a fence-post up the collector to shift a tube over.
The most challenging ones, Ive had to use a belt sander and draw-file the weld on the head flange, to change the angle the tubes leave the head.
I guess I've just been lucky lately, but I have installed dozens of sets of every brand since the late 60's.
Bashing a tube should be your very last resort, after those mentioned above.
Bashing a ceramic tube fractures the ceramic, then rust follows, inside and out.
Good luck