I used to get more than one lot of CM in my cycle when it was a longer one of 35 days+ so the MB helped me to figure out which one was the real one due to the ferns being nice and thick. When they were thinner or mixed with pebbles then I could disregard it.
I also used OPK's sometimes too but that was when I was trying Shettles TTC a girl so I was trying to get in the BD before O.