FYI, Atlas Code 55 models 8-foot ties and Micro Engineering Code 55 models 9-foot ties. If you are doing 1950s or earlier, you would be more prototypical with the former, for most railroads. In my case, for example, I am doing Tehachapi in the late 1950s, and that portion of the line was still using 8-foot ties back then. I believe that the tie spacing is very slightly different between the 2, as well.
BTW, the above fact also bears on the idea of mixing Atlas switches with ME flex.
For what it's worth, I would go with ME when doing a modern layout. The flex track "stiffness" is about comparable to that of Shinohara code 70, which was "best practice" back in the early 1980s. I laid a lot of it, using the techniques suggested in the MR Clinchfield book. Their procedure involved pencil center lines and track nails, which were then nudged back and forth for final alignment. Then you did the ballast and clipped off the tops of the track nails. Still a good technique, in my opinion, so long as you drill out the dimples/holes in the ties to avoid splitting.
MH