I think it depends on what your expectations for the plots are.
If putting in a perennial plot like clover, chicory, alfalfa, etc. then the more weeds and grasses that you take care of early on the better. You will be sorry if you don't kill the competition for a plot like this. Cool season grasses will be the main problem. You can't hurt them by mowing.
But, for a strictly one-shot fall annual plot like grains and/or brassicas, you can get by with less prep. You should have a fall bounty of the annuals you plant before the cool season grasses can recover.
In a clover plot you can generally control most broadleaf weeds by mowing at the right times. The reason you see weeds the next year is mostly because of the seed in the ground that germinate after you kill the plants with gly. It can take a few years of work to exhaust the seed bank at any given spot.