Now the dust has settled regarding burn in and the fact that quality manufacturers do burn their products in, here is the proof from Weiss :
5.3 Manufacturing Information
The manufacturing process of the OP1-BP is targeted for best performance, consistency and reliability. After the four layer printed circuit board is assembled and soldered the output stage heatsink is attached with a high-performance adhesive for a reliable, low loss connection. The fully assembled amplifier circuit is now exposed to several thermal cycles. This burn-in procedure ensures very stable operation afterwards, particularly regarding DC precision and the various bias points. Subsequently offset voltage, input bias current and class A output current are trimmed with the amplifier fully warmed up. Extensive testing of each specimen is carried out for several critical parameters.