Typically, the bullet will center itself in the sizing die. However, if the top punch is slightly off center it WILL cause the bullet to size off center. This is NOT helpful to accuracy.

Instead of using the set screw to hold the top punch in place ( because the hole for the top punch MUST be larger than the top punch to allow you to insert the top punch into the sizing machine by hand, use of a set screw to secure it cannot help but push it off-center) put a smear of sizing lube on the surface of the top punch that seats against the machine. With just the "stickiness" of the grease, the top punch will be held in place but it will also allow the top punch to "self-center". I have been doing this for years.

Dale53

Another way is to size a bullet nose first then using that bullet set up your top punch as follows. 
Grind a flat on the stem of the top punch where the set screw contacts it.  Then open the top punch nose with a drill.  Put the nose punch into the lube sizer and run the set screw in to hold the punch Take the sized bullet and put it into the sizing die a bit.  Put five minute epoxy into the top punch and lower it onto the nose of the bullet and let it set up.  Lube the nose of the bullet for easy release.  If you put the top punch into the lube sizer with the set screw hitting the flat every time you use it with that bullet you should get straight line sizing. 
mes