Although you may have the option to shoot in b/w mode you may want to consider not using this option. Why? You might ask? Because once you throw away the color information it's not coming back. This will color information is extraordinarily useful when you are processing your image into b/w. Having said this there are times when you can't wait to post your images online for a blog or onto Facebook and you would really like it to be b/w this is perhaps a good time to be shooting in b/w mode.
(Please note that if you are shooting in RAW your color information is still available)