This may not be an exact answer to your question, but I had the same problem with my 1:12 78 JPS Lotus.  My problem was not so much a seam but a step on the front tires.  By step I mean that one half of the time was slightly larger in circumference than the other side.  Although it is only a fraction of a mm it would have been hard to get right. 
  
However, while getting reference pictures for my build I came across a very clear, head on, sitting still, picture of my car in the pits with new front tires.  There was a definite, unmistakable line around both tires that looked like a seam to me.  Knowing that I was going to show this car in different IPMS shows, I scanned that picture and printed it.  I took it with me and displayed the picture with the model along with a high-lighted note that said "Please note seam on tires".  (I have been burned before by IPMS judges specifically for tire seams).  I won a gold medal at this show. 
  
My point is, I understand wanting to get rid of that seam.  It is good, basic skill level modeling.  But I have seen more than one picture of new tires with what sure does look like a seam down the middle.