This is the quickest recipe for a crooked gas check. I still use my 450, it's a good sizer, but it is limited in what it can do. I like to seat checks by hand if possible, or use the Lyman "blocker" to seat checks if more force is needed, then size. Dropping a check in the die first might work if your shank is small enough that a gas check just falls on.
I've ran bullets forward and backwards in Lee dies, they crimp gas checks on just fine. I'm as ham fisted as they come, and I can't even remember the last Lee product I broke. Anything I broke more than likely got dropped and stepped on. My sizing dies are plenty round. I've never put a micrometer on them, but my bullets measure round from a calipers, under .001".
I'm still confused why people have problems with the Lee dies? Whether its a Lee push through, NOE push through, Lyman die, Star die, or anything else. You push a bullet into a die, and it gets sized/crimped onto the shank. Where is the problem?
I'm sure the NOE sizers are good, and probably the better way to go long term. The Lee dies are good dies though.