After I have toned and am positive I will never get my 16 x 20 prints wet again, I trim the edges ever so slightly w/ an xacto knife and ruler. The dimensions of the prints I'm working on right now only allow trimming of two sides; works as well as trimming four.

After the prints are trimmed, I place them individually between 2 pieces of 4 ply acid free board in dry mount press at 200 degrees for a few minutes. I then put them under a print flattening plate I got from light impressions to cool though I have used a heavy book in the past. A teacher explained that trimming the prints releases the surface tension of the dried emulsion.

It's super easy and my prints are flatter than the unexposed paper was straight out of the box. A 1/32" trim or even less has worked for me. hope this helps.