If you don't ask, you don't learn.
Some brands you will notice a ridge or lip on the thread, so you can tighten the nut untill the washer hits this ridge, the nut wil stay tight and that is the designed state for the rubber.
Brands like pro-comp leave you the option, and it is just that. Provided you have the nut fully on the thread the ryubber will be tight, you can compress it further to your own taste really, for example if you notice a 'fluttering' in the wheel/axle in question then tighten the rubbers up a little, if you extreme off-road leave them looser to allow for flexing. just remember that for on-road handling the looser the rubber the less control the damper will have.