Very short answer, no, the importance of the so-called medieval warm period has been greatly overstated. This article that I found via a Wikipedia pages seems fairly well sourced and brief, http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/070.htm. As far as I know, the consensus among historical climatologists is that the phenomenon was not widespread or significant enough to affect major, large scale historical change. I once had to sit through, what was, as I'm sure you can all assume, was an exhilarating presentation by an expert in the field. If anyone is interested, speak up and I can provide some citations for this that are full of numbers and diagrams and things I generally find less interesting.