Tom,

This isn't your amp, where  a few thousand volts are floating around. 
While you should always be careful messing around with this stuff, look 
at  it this way:  If you turn off the supply and unplug it from the 
wall, and your hand brushes against the a.c. plug, will you get fried? 
If you are super paranoid,  unplug it and measure the voltage at the 
plug, which will be zero, and proceed.

I do think you should tighten this thing up.  I'm not sure it is even 
the "official" ground lug.   On the inside it is holding a small thingee 
  with wires connected to it.    I'm not positive, but it may well be 
where the
third wire of the a.c. line connects to chassis  ground.

If it were me, I'd tighten it up and ground to one of the cover screws. 
    And since you  don't know you're own strength, just tighten good and 
snug, you're not torquing down head bolts on a Mack truck.

73, Steve KW3A