www.shaolinhouse.com
I got my dummy from them, and just by rotating the dummy arm by 180 degrees, you can choose to have offset or level arms.
To be quite honest, the only reason why they should be parallel, is for the left bong-sau. If the arms are offset, then when doing the left bong-sau you twist your arm and the bong-sau isn't in the proper form. If the arms are parellel, then your bong-sau's are in proper formation.
I know that some pictures show Yip Man himself using offset arms, however, later in his life he noticed the difference in the bong-sau and this was his idea to have them parallel.