I replied to this thread way back on the first page or so, and have watched it progress, get kinda nasty, and so on.
This only applies for those who have a distinct, limited time calving season, and are not year round calvers.
Spending time every day checking cows and observing those that come into heat seems to be way too much time consuming to me. I have a herd of 50 cows, row crop 150 acres, bale about 1200 big bales for myself and others a summer, up to 8000 small squares a summer, maintain 7 different lawns, all in addition to working an 8-5 job. I have enough time to check my cows about once a week for a good portion of the summer. No way could I check them every day to see which cows came into heat after my 45, 60, 75 or whatever the magic number is breeding season. Even if I didn't have a day job, I still wouldn't do it. There are much better things to do with my time that are much more profitable than heat check my cows every day.
I guess everyone must view their time differently, and you must use it the way each person sees fit. I would still think that culling cows based on an observed heat after the calving window will lead to culling some bred cows. I have witnessed bred cows stand and allow other cows to mount them, and even a bull on a couple of occasions. It is a rare occurence, but it does happen from time to time. Maybe the cow was just to lazy to move out of the way? Preg checking is not hard to learn. I took a semester course in college on how to do it, and was able to tell if a cow was bred 2 weeks into the course. If you are pregging cows that are 60+ days bred, it is really pretty easy. I know I have saved myself a lot of money by pregging my own, and culling open cows rather than feeding them all fall and winter.
Just my 2 cents.
In the end, to each their own, and whatever works for your own operation.