One usually does not (and absolutely should not) have any flare in a bottleneck rifle cartridge. Ever.
For me personally flares don't apply.....having NEVER flared one, personally. However in the 45 years I'm been reloading, I've noticed
many do do it to load lead "boolits", since the usual chamfer isn't enough to prevent scraping lead off on the way in. The Lyman "M" die was created as a better way. While I've yet to load my first lead rifle bullet, much preferring jackets, I understand the monetary reason for the practice.
The "other reason" some might think they want to flare is using a bullet feeder. I think the "M" die is the better idea here too, for keeping bullets upright around a progressive merry-go-round. Instead of a flare the first 1/16" of the neck is expanded just a little more, making a pocket for the bullet base to rest in. (such doesn't work as well for boat tails, nor usually needed). Any crimp I personally do is just to straighten and firm such. If you've never loaded rifle on a progressive with a bullet feeder, I can't see where you would ever need to even use an "M" die on jacketed bullets.
I might add....never tried it yet, but I can see, on a boat tail shape, that a slight flare might stabilize that base better on a progressive merry-go-round than an "M" die pocket could. Makes sense to me.
BTW......Am I right in thinking not all Lee Factory Crimp Dies are designed the same? Some are more like a taper crimp and adjustable, some are not....more a collet pinch?