It's relatively easy to throw in a new set of springs and be happy. However, an alignment after lowering the car is essential to keeping a well handling vehicle. Lowering the vehicle will lower the center of gravity of your vehicle and aid in cornering, but it changes the dynamics of your vehicle. Your front control arms and tie rods are no longer parallel to the ground and possibly creating a bumpsteer condition, the strut travel distance has been reduced, tire fitment may become an issue, and these are just some of the obvious changes. In addition, if you've increased the spring rate significantly (an approximate 650+ lb front spring) your stock struts will not be able to withstand the additional forces. One sign of your struts dieing is if you begin to see fluid build up around the exterior of the strut. Based on my experience with the Koni struts, the red Koni's are a stock replacement, but will function close to the 650lb front spring rate. The yellow Koni's are a better choice, but are needed above the 650lb spring. Some shops also work with different strut manufactures to tailer the valving to their suspension designs and spring rates. Ask the companies about the best spring / strut combo when ordering.
Changes to your front end alignment are made by altering your caster, camber, and toe settings. Caster and camber are easier to alter if you use an aftermarket caster/camber plate mounted above the front strut. Some plates will allow you to alter the caster independently, while others will come fixed at a preset maximum. To the left is a plate that has a fixed caster setting of approximately 3 degrees positive.
CASTER: Positive caster is the rearward inclination from vertical that a line passing through the upper strut mount and the lower ball joint creates, when viewed from the side of the car. Leaning backward toward the rear of the vehicle is positive caster. Pictured is the front right wheel.
CAMBER: The angle that the wheel differs from vertical when viewed from the front of the car. Inward toward the car is negative camber. Outward away from the car is positive camber. In this photo, you can see the slight inward inclination near the top of the tire.
TOE: The difference between the horizontal distances of the centerlines of the tires, when measured at the center height in the front and back of the tire. "Toe In" means that the front tire edges are closer together at the front than at the rear. The tie rod on each side of the steering rack is used to alter the small changes.
BUMPSTEER: Lowering the vehicle changes the tie rod angles between the steering rack and the steering knuckle. In turn, this effects how your toe setting changes as the suspension travels through it's up and down motion. To obtain the proper bumpsteer, you need to limit the change in the toe setting through the suspension motion by changing the positioning of the tie rod. There are three methods of changing that geometry:
Bumpsteer kits (which replace the tie rod end with an adjustable piece).
Steering rack bushings (which attempt to raise the steering rack slightly).
Swapping spindles (94/95 GT/V6 will change the geometry without pushing the track width out substantially. Spindles newer than that on a 87-93 Mustang will change the track width and modifications will be necessary).