I would think the forces applied to the bars would vary greatly depending on how you tie the bike down. I, myself no longer use a blocking devise between the front tire and the fender to keep the front forks from compressing. I stopped because using the blocking devise transmits very hard repeated impacts to the steering head bearings and will wear them out prematurely. Using one of those blocking devises would also greatly increase the stress put on the handlebars if that 's where the tie downs are attached.
You can still travel long distances on very bumpy roads with the bike only loosely tied down if you will do 2 things: (1) make sure the front wheel can't turn sideways and (2) make sure the tie downs can't come loose. Regular tie downs can slip (especially if they get wet) but that can be easily remedied by tying a slip knot right at the locking mechanism. The hooks on the tie down ends can bounce off if the bike is loosely tied but that is easily avoided be replacing the hooks with caribeeners or taping the hooks after securing the bike.