What's important is that your racquet cannot move during stringing. But don't fix your racquet too tightly. Adjusting the weight Before you actually start to string you have to know how hard you want to string your racquet. Ask the person who previously used to string your racquet about the tension so you get an approximate value. As you now have the total control over your own string tension you can easily try different tensions, check out how you play with half a kilo more or less.
Less tension provides more power, more tension provides more control.
The more power, the less control. Just evaluate the speed of your swing - the faster your swing the more tension you should use. Stiff racquet frames, above all Widebodies, and also oversize racquets should be strung with a higher tension to equalize their power. With a drop weight tensioner you just have to move the weight to the desired position on the rod. With spring tensioners you adjust the tension using a screw and with electronic machines the tension is entered digitally.