I'd say Dean definitely had a thing for brunettes. The blondes for Dean do seem to represent sister/mother types. Sam has Jess, but I think for Sam also, as for Dean, every blonde is their mother who has to be saved. (I guess that's a fairly Freudian take on it).

The Cassie=Sam and Jess=Dean parallel has come up in discussion before and it actually makes sense to me (and I don't even think Sam and Dean are doing each other). But that they'd seek out love interests with qualities that remind them of what's best about each other -- growing up as close as they did and being separated at the time Sam met Jess and Dean met Cassie, there's gotta be some kind of displacement issues going on.

Or, y'know, we're reading too much into it. *g*