theres probably a best practice, but I wouldnt stress about it. I pinched a top one time that had like a half inch of stem and leaves that probably spread about the diameter of a nickel and for shits and giggles dipped it in some rooting horomone and it ended up rooting and growing. keep in mind that it was rooted and growing, just very slowly, but just know that if you use a good rooting solution and good medium in good conditions just about anything can root and live....just using larger more developed branches is going to save lots of time in veg.