[Joe]

Sub-domains are best used to separate large categories of different but related information within a site, for example, if you had a site about pets one sub-domain would be for cats and another sub-domain for dogs and another for fish etc. each sub-domain would focus on one large category of information.

So, my advice, would be to do things naturally, and not intentionally try to over-optimize.

I find that if you do things naturally, Google will pick it up, and you will be rewarded well. My point is, don't go crazy and creating hundreds of subdomains and such.