I have never applied pre-m to my home lawn. I have only sprayed broadleaf control (a blanket) 2 times in 8 years. My lawn gets no annual grasses at all. Only 2 or 3 dandies a year. I live across from a big park loaded with weeds and crabgrass and the neighbor to the north has tons of weeds. My lawn is not real dense. It is very old and needs aerated a lot.
I think that too much herbicide causes the soil to build up some sort of immunity. Trugreen sprays Trimec 4 times a year on their lawns (here)and they just can't get long term results.
I am seriously considering not using pre-m next year and going with a post emergent.
How cost effective is that, though. I pay $10.50 a bag for fert/pre.