Thanks for advice from all of you,
So I guess filling rear is just one's preference or for rear passengers? If that is the case, I wouldn't insist having rears.
The main goal of replacing the system is to have up-to-date technology - Blutooth/Phone connectivity. I first wanted to simply replace the HU but it turns out the fantastic factory premium audio system called "DSP" on my 2000 BMW M5 does not cooperated with aftermarket HU at all. I looked for every possible way to keep the factory speakers but I concluded it is not possible. Well what the hack I am upgrading the entire system.
Another question: what would be the appropriate level of labor charge for installation - considering it's BMW. I contacted a couple shops around me and they all charge double for BMW due to complexity of the existing factory system. The lowest was $450 labor only. Google review was good and they seemed they know what they are doing.