Try a Lyman "M" expansion die, it works well for me. It leaves an even "ledge" at the top of the case just big enough to set the bullet on straight, without making the mouth of the case too big. You can get them from Midway or some of the other mail-order places for $10-$12. And as David said, use the proper seating plug. A seating plug for roundnose bullets let SWC bullets tip, and the seating plug for SWC bullets will let roundnose bullets tip. Often a call to the manufacturer of your dies will get the proper seating plug sent to you free, or for a couple of bucks at most.
Another thing to try if you are using a single stage press is to seat the bullet halfway, then rotate the case 180 degrees and finish seating the bullet. DON'T do this with a progressive as you will end up double-charging the cases with powder.
And if you just can't get the bullets in straight, you might see if the LEE "Factory" crimp dies will straighten things up during crimping. I understand that this die has a re-sizing ring in it, and it might be able to iron out any bulges in the case - straightening the bullet as it does.