Man, when I was in school (college), we had to learn all of this and understand/caculate potential seek/read times on a DASD array, given a specific sector/cluster size, number of cylinders, seek range, and data transfer rates. This was so when we were designing applications or databases, we could tool them to match the performance of the primary hardware upon which they would run. These days, it seems like they don't even teach this stuff any more.
But yeah, Whitey and North are right on the money. My advice is to leave it at the default value. Unless you have enough knowledge to understand the advantages of changing it, and you know that you're going to have a concentration of a certain type/size of data that justifies changing this to your advantage, the default is probably best. There's a reason that it's the default...it's already been determined that, for most people and the completely random variety of data types and sizes they're dealing with, a 4K allocation unit will give you the best overall performance/space usage ratio.