I have given a slight warm flash to give more natural look in skin tone.
second enlarger , flash the whole paper, then put into standard enlarger and print
It is fine anywhere in the light train except under the lens or between the lens and negative. In fact, with sheet film, there is an anti-Newton Ring layer as a coating on the back in some films. It was recently eliminated due to scanning problems.
This layer on the back of sheet films was intended to reduce Newton Rings and also aid in retouching.