I top my autos when the stems show thickness, and have tight node spacing. I also do this before day 21 +- a day. If they are taller spindly stems, I do not mess with them. Once I top I just LST afterwards.
Here is a FIM on my current grow, and it truly was a FIM not purposely done! If you look close you can see the original center of the plant and she has grown what appears to be a center cola again! Center cola is small for for now, but she has at least another 4 to 6 weeks of growing. She did spread nicely and has quite a few bud sites forming. This is a Zamnesia WURLZ F1 Auto.