It depends on the size of your room and the amount of bass you want. For the majority of users, a pair of Fives alone produce plenty of bass.

I agree with @GuitarSuperstar ‘s assessment.  I suggest just get the Fives to start with.  You can always add a Sub later if the Fives don’t give you enough bass.  I suspect you will be happy without, but only your view and preferences matter in the end.