bullet mold question

[Mr. 16 gauge]

When I first started casting (back in the Jurassic era), I was told by an experienced caster who showed me the ropes that at the end of each session, you should leave the molds with bullets in them....supposedly this helped keep the two halves "even" and prevented warpage, or some such thing. Last year I bought a Lyman shotgun slug mold, and the specifically said in their directions NOT to do this practice. This is my first Lyman mold (all my others are from LEE....their directions don't say to do it or not to do it).
I curious as to do the bullet casters here leave a bullet in their molds or not at the end of each casting session, and why or why not?
Thanks in advance..........