He was certainly using just 5 back in 1991, though he could have later moved to 5 (or wanted people to think he used all 5 by specifying them for his signature model). It partly depends on the position of the string claw. If it's set quite near the trem block then the springs will be under less tension to start with, though the force required to move the trem arm once you get past a certain point will still be greater than with fewer strings.

But just because they are fitted, it doesn't mean they were used. My Dave Gilmour Strat came with 5 trem springs fitted, but DG only uses 3.