I actually just completed the exact upgrade you are talking about. Front BSW set and the BSW sub. It was a hugge improvement over the stock speakers. The rear speakers are primarily fill anyways so I chose not to upgrade them. Acoustically speaking the better speakers you put in every location the better the overall sound, however, depending o your listening preference it upgrading only the front sound-stage works out well.

I listen to a wide variety so having the softer original speakers as a low level fill in the rear actually gives it a warm sound to accompany the crisp new front end. Sound can often be very subject, especially in a vehicle so it makes sense to try it and then take the next step if you feel the sound is not right for your taste.