I take large screw driver & leverage off one side of the lower mount bolt, against the other side of the pulley bolt, relieving the tension given by the spring. Then quickly slip the belt on or off the pulley.
Or a closed end wrench on the pulley bolt by itself & leverage against the spring tension. (better choice)
If during this, you realize how painful it would be "if" you fingers got caught between the belt & pulley when tension is returned to the belt, you're probably doing it correctly.
So no, the mount bolt should always remains secure unless the tensioner itself is being replaced.
Welcome to the Forum murphy!
If you need a belt/pulley diagram, just ask & mention which engine. As many times as I've replaced my serpentine, I still tend to forget the proper routing. (especially if beer is somehow involved)