That's always a quandary, which version do you target? Pro vs Standard, V2003 / V2007 or newer. The newer versions have updated object models which general mean more efficient coding but also restrictive applicability. As time progresses, the customer base using older versions dwindles...slowly. I think the average Visio user tends to cling to their version, for better or worse, until some circumstance moves them to a newer version. Sometimes, the new object model just enables you to do some things not possible with older versions.
Always a difficult decision.