Equalizer setup ?
I have a different style hitch, so I'm not 100% positive, but most hitches that use washers for head adjustment use them as spacers to control the amount of front to back tilt you have. The farther back (bars pointed more to the ground) the head is tilted, the more leverage is applied when placed under load assuming you don't change anything else. The more leverage, the more weight is transferred. So , to answer the question, the washers are there to fine tune tension. Before you start adding more, I'd break out the manual and take some measurements hitched and unhitched so you have something for comparison.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition