Cosmos

My Cosmo plants are coming along nicely. A few are this size now.

Should I be pinching the centre out when they get to this size?

 

Yep Kriss....pinch centre out at those top large leaves.
The other thing is to hold back on the watering....cosmos like it on the dry side

 

Should I wait till they all get this size before pinching?

 

Yep, got a package of seeds for that. they grow quickly. I will be putting them in a long type planter soon (the seeds) and let them do their thing. I don't fuss with them

 

No Kriss, pinch those as they get to that sort of size. Don’t be afraid to do it.
As an experiment or a bit of fun, you could leave a couple of plants unpinched so you could see for yourself the difference pinching makes.
I pinch cosmos 3 or 4 times. Plus when they are actually planted out....mine will be in 1 or 2 litre pots by then....I pinch again.