The larger the screwdriver the better, it shouldn't have any room to wiggle. And in that case, strings can stay as they are. If you use a smaller screwdriver (some people even use coins), because there is nothing better handy, detuning a little makes things easier. If you want to drop action just a little for all strings, adjusting the middle screw will do unless the two larger screws are already higher and your bridge is slanted forward which it shouldn't be.
Send a pic of your bridge and we can tell you what to do.