It's better to top it early. Because you end up with a shorter plant with 2 tops. Or even better way is to bend it gently so you can tie it low to the grownd. If you do this every week or so they will grow along the ground with a huge yield. I'd top it after the first 7 nods grow.
Then if t needs to be topped again do it about 3 weeks before flowering. Most likely that would be around the end of Augest.