best time to top autoflowers is when their in pre-flower stretch, I know kush-t did a great example of this with his auto mazar and it does give for a nice even canopy, because they are in pre-flower stretch it doesn't slow them down as this is probably the fastest rate at which the plant is growing
you can pinch them 7 days after showing sex as pre-stretch normally lasts 2 - 3 weeks
some people like to lst to get even canopy but its just down to grower preference
a good question is does it produce more weight?
the fact that you topped the main cola normally makes other budsites crave the attention so instead of having a Christmas tree like structure your get a nice even spread of top colas which should all fill out nicely
Seymour