You honestly can top whenever there are other nodes below the one you cut off. Your best bet is to wait a week or 2 into veg. And give it a week or so to veg after you cut. But you can top in early weeks of bloom if height is an issue. (Not as good as topping in veg IMHO). You should just top one and make note of how it grows afterwards and that will help you decide when to top next time.
I leave the bottom two nodes alone ... but im not topping for space limitations either. If its to save space by topping almost every node, start early. If not, wait till you get up to your third top node.