It's been a few years now. That's plenty of time for massive herds to wander out of the city and start roaming the countryside like locusts. Having a farm out in the middle of nowhere may seem easy to manage if you're only dealing with a few dozen at a time, but when a herd of ten thousand comes shuffling through, you're pretty much fucked.
We did see that one large herd out at some random rural farm in the episode this season where the Abraham/Glenn group ran out of steam at the end of their journey to get Professor Pants On Fire to DC.
But out in the middle of Montana, it's safe to assume that there are no huge herds. It's not like the population of Chicago or Minneapolis would wander off to the Dakotas or anywhere else in the northern plains.
And if there is a large herd somewhere in Montana or South Dakota or wherever, there's likely only going to be one. If you run into them, you can just go 50 miles away and feel relatively safe.
The problem with going too far north though is the winters. They don't have the luxury of storing up food over the summer to prepare for winter. Iowa/Nebraska/Indiana/rural Ohio or Illinois still sounds like the best bet to me. Actually forget even rural Illinois or Indiana. The further west the better. Find some nice big wide open farmland where you can see for miles. The further west the better. Keep going until the walker numbers noticably decrease.
Just at random, I looked up populations in Kansas ranked by county, and there are counties out there with less than 2000 people. If our group can take a prison, they can clear 1500 (less actually, since many would be beyond the point of reanimation) walkers to claim an 800 square mile county. Or hell, go one state north and there are a bunch of counties in Nebraska with less than 1000 people. Head to the places where the population density is less than 1 person per square mile.
Though it does seem logical to assume that isolated households in the middle of nowhere would be rather survivable compared to city dwellers.