Your 'roll' is a crimp. Unless you're loading hot ammo, you don't need any crimp. It's detrimental to accuracy. If you are loading hot, use just enough crimp to hold the bullet in place. It's a trial and error thing, but it doesn't take much of a crimp.
If you plan on loading .38's, use .357 brass loaded to .38 velocities. Eliminates the lube gunk ring in the cylinders. The gunk ring is not a big deal though. It comes out with normal cleaning. It does have to come out if you use .38 cases then want to shoot .357's. Loading .357 brass to .38 velocities makes it go away. Use a different bullet shape or a different brand of brass for each. No need to worry about what ammo you have. That's not a big deal either, but having a .357 go bang when you're expecting a .38 will throw you off.
