you don't want it loose because that would yield unpredictable handling (it would feel like no sway bar, then BOOM! SWAY BAR IS WORKING as you turn). You want it firm as in no wiggle with all wheels on the ground, but no tighter unless you want preload.
Understand how a sway bar works... it's a sideways spring that connects the left arm to the right arm. As you go into a turn and load on one side of your suspension, it will transfer resistance to the other side. The bigger and stiffer your sway bar gets, the more load is transferred. The downside is that as you increase the size of a sway bar, you remove the independent nature of the control arms and turn into a beam suspension which will make the car more likely to snap and slide instead of lean and grip and also act skittish on uneven/bumpy roads.
If you're still with me then your logical question is why do you even want swaybars?? (why not just stiffer springs). The answer is because really stiff spring rates make the ride horrible on a street car, and are not easily adjustable like swaybar is on a track car.
02 BMW X5 // 97 BMW 328iC // 89 325is w/ M50 swap // 06 Audi A3 w/ APR stage II