You may wish to consult my two recent contributions to this website on scanning with SilverFast where I discuss the program's colour management capabilities, including its automated IT8 profiling feature: Old World Joins the New
and Plustek Film Scanner Review
. The advantage of using custom profiles for your scanner is that you should get more accurate results, because no two scanners even of the same model behave identically. Canned profiles aren't necessarily bad, but well-made custom profiles should generally perform better.
Please be aware that Kodachrome can be a difficult medium to scan very accurately in certain scanners, including some Nikon models, regardless of the profiling solution.