Its always good to have positive slack. Tools like DC and magma, will try to optimize best for meeting Zero slack. More positive slack can be achieved by compromising the area..If yor design is not area constraint, you can have positive slack too...
In general, If you have excess money its good and you will still survive with required money..Hope you got it...
positive slack is always desirable. in present technologies where we are facing the power problem this plays a more role in reducing it.
we have the concept of Multi vt libraries. The cells with high Vt are placed in the non critical paths(slack is posittive) and there by reduce the leakage current.
It is always good to have positive slack rather than having zero slack. This is because in further process of chip design (like in backend) there may slight timing variations which can be accomodated if there is a positive slack.