Blade bolts do not self tighten. If you have difficulty you are tightening them too tight to begin with and fail to clean and lubricate the threads. A simple box end or 8" ratchet will tighten them sufficiently. Small couplers do not lower the Pressure, they lower the Volume.
Rubbish
If the blade has a round hole then it will self tighten
So the star shapes found on domestics naturally can not tighten and neither can the ones that sit in a cradle held down with 2 small bolts like some of the older Murray & JD mowers
However all of the current crop of of commercials do self tighten .
I have 6 Toro Z masters in my repair run
3 of them are used commercially & 3 are residentials
The residentials are no problem because I see them every 2 years os so for a service when I sharpen the blades
Two of the commercial customers get me to swap their blades every week & they are no problem
THe final owner cuts street verges on country roads & runs his blades till they are worn to stumps
His bolts have to be cut off
I always use a tension wrench to do the final tension on blades so none of them are over or under tightened