If you have a forward pattern shifter, then yes, you need a reverse pattern shifter. Although you will be able to drive with the wrong shifter, you will not be as safe or able to shift quickly or smoothly through the gears. Some shifters can do both by removing a piece or adding a small piece in the gate area. In a reverse valve body, you shift away from reverse from low to high.
I think B&M pro stick come with both gates, or i know for sure the reverse gate can be purchased seperately and Topspeed is correct, it's unsafe unless you have the reverse gate installed.