Being in Worcester, I've always considered us the beginning of Western MA. I drive west of the city and it starts getting very rural and the forests get more expansive. I drive east and it's more congested. Politically the state demographic changes about 2 towns over from Worcester as well.

Technically Rutland is the geographical center of the state (they have a tree that marks the center in town, worth the stop and picture) and I think that once you get 2- 3 towns west of Rutland you start the beginning of Western MA.

84 and the CT River are the best boundaries to really delineate where the west starts. The proposal to make WMA its own state always started at that marker.