commercial fb papers
i work with ilford fb most of the time. primarily mgiv. ive seen a million posts on this. firstly, the bit about trimming the edges is true. the small waves along the length of the page seem to occur because of tension at the edge of the page. i dont know how they make their final products so i can assume a dozen reasons why this works but it just does. You only need trim less than 1/8 of an inch for it to be effective. you do so after you have processed the paper and have dried it to the point it no longer has a wet surface. but the paper is still saturated. i just lay it on a large cutting board (image up), use a straight edge and a very very sharp knife. There is technique to trimming wet photos. for example very light sawing at the start of the cut to ensure you are thru the paper before continuing or you will wrinkle the corner of your print, etc, etc, etc. I have never tried trimming an unexposed sheet. I hate trimming paper under a safe light.
second, you can iron prints. what ever method for iron or heat drying you use, dont have anything in contact with the print that can transfer any residues or has any texture you dont want. the paper will pick up anything you touch to it, and will take on textures of fabrics and such you use to isolate the print from the iron. but ironing is a process to use if you are in a hurry, as is drier drums, heated blocks, etc. prints will stay flat because when they cooled and/or dried they were in a straight position. if you use anything with heat you should cool them with time and pressure method.
Time is the best thing for fb print paper. air dry them until they are starting to curl and put them under flat weight. isolate them as mentioned. do not use things that absorb moisture. do not start this process if the print has not started to curl or you have rewetted portions of the paper. paper curls because it doesn't dry uniformly. the emulsion dries at one speed, the paper does another, and portions of the paper dry differently than other portions, also any hardeners such as hardening fixers can exaggerate the problem.
i dont print more than a few 16x20's at a time. so i usually iron them and then put them under long term pressure. i live in florida. take a perfectly flat fb print and put it in the car and you get to start over again. So humidity is a factor. not just wet. remember that when you choose your isolation materials. if they hold moisture, they are maintaning a humidity level in your print. take it out in the ac and presto rollo.