Hi Duane,
You are mistaken. The SiteTitle control does in fact render as an h1 with the class siteheading thus an easy selector is h1.siteheading.
<h1 class='siteheading'><a class='siteheading' href='http://demo.mojoportal.com/Default.aspx'>mojoPortal Demo</a></h1>
also the link inside it is using the same class.
As far as what is required, nothing is required unless you want to style something. These are fairly important though:
.leftside { }
.center-nomargins { }
.center-rightandleftmargins { }
.center-rightmargin { }
.center-leftmargin { }
.rightside { }
because these will be assigned programatically according to whethert left center ritght panles have any content so implementing these allows you to have a site that adapts from 1 to 3 columns automatically.
Other than that the selectors used are based on what markup is there and what you want to style. I could make up almost an infinite set of selectors depending on what I want to style though there are selectors that are used fairly commonly across existing skins to style things that are very common to apply style to. The markup provides lots of class names but you should not think it boils down to key value pairs of classes and style rules such that the selectors can be enumerated once and for all as a definitive list. Whatever selectors you use must be something that selects something that exists in the markup or it has no effect.
Basically you decide what you are trying to style you view the rendered markup and see if it has a class on it or some parent container above it has a class you can use as part of the selector. If I want to style a link in my content and the link has no class on it for example I can use a container element's class .like .modulecontent a {} will style all links inside any element that does have the class modulecontent even nested ones.
The ones with id selectors in the layout.master like #wrapheader are arbitrary, you can use that or make up your own and wrap it around something in the layout.master to give it some style.
Hope it helps,
Joe