For a street car preloading isnt neccessary at all, set the preload to zero. That means adjusting them so that the spring (while the coilover is off the car and fully extended) is held firmly between the top mount and the adjustable spring pirches, but not firm enough to compress the spring at all. Doing them up by hand is all you need do.

With the type of shock that allows you to adjust the legnth of the unit seperatly to the spring preload, it is best for you to leave the prelad at zero, this is the reason why.

Preloading is a very useful tool to adjust where abouts the shock lies in its stroke when the car is sitting at its static ride height. This determines the maxium amount of negative stroke and positive stroke the shock is capable of.

Say you want to lower your car a heap, doing so with just the adjustable spring pearches will reduce the shocks upward stroke. If the car is bottoming out like this, you will need to adjust the actual length of the shock, by the height collar, this allows you to have more shock travel when lowering, to avoid bottoming the shocks out.

A street car should never be reaching its minium and maximum travel, if it is, the springs are too soft for the application, making the AMOUNT of stroke needed of the shock too small.
(shock may have 10cm of stroke (minimum to maxium) but the car wants to bounce up and down more than 10cm. In this case it will be likely that you will experiance both bottoming and topping of the shock, or very extreme and frequent bottoming or topping).

OR

The shock travel range may also be to low in either bump or droop travel, which can be altered with shocks that have seperate length and spring perches.

In your application, having the preload at zero, and adjusting the length for ride height changes, will allow you to keep the shocks well within the ranges of the shock travel you will need for the application.

A very common misconception with alot of people is that adjusting the ride height by the spring perches, changes the stiffness of the unit. This is incorrect, the spring rate, and damping characteristics of the shock will be exactly the same no matter where the spring perches are located on the body. The only thing effected is shock travel.

If your have coilovers where ride height changes are made by adjusting the spring perches, and those coilovers arnt bottoming out at your chosen ride height, then there would be absolutly no benifits in going to a coilover which allows this extra adjustment of shock length, another common misconception.

If i were to severly lower my car, i would definatly go for a coilover which has seperatly adjustable length due to the adjustability of the availiable shock min and max travel. Bottoming out is extremly damaging to the shock, and in the case of coilovers, expensive! not to mention that bottoming shocks during cornering is a very dangerous situation most likley resulting in snap oversteer or undesteer, depending on which end of the car is bottoming.