I dont pinch the tip off cosmos, so Ill be interested to see if I should. At the moment mine are too fragile anyway .

I've never bothered nipping out Cosmos. They seem to break into multiple stems quite naturally.

I pinch them out when they have about 3 sets of leaves. They can get very tall and leggy especially if you start them off inside. But when they get going I have flowers until october.

I also usually tip-pinch them at 3-4 sets of leaves, mainly to make them more wind resistant. Otherwise, they pinch themselves the first time they bloom.