Personally I like to top at around the fifth node, sometimes sixth. I almost always top early as it provides an excellent support structure for your plant early on. Here is a picture of a plant that shows an example of when I usually like to top:
Top after the 2nd or 3rd node, if you wait until the plant is about 12 inches tall you can top the main center and dip it in some rooting hormone or cloning gel and have another plant ready 2-3 weeks after your first harvest. the node is where the branches alternate, each set is opposite from the other, in a cross pattern from looking above.
Anywhere from the 4th set to the 7th set is perfect. If its a really healthy fat plant I top younger. If its a runt I wait for it to get a little bigger and stronger before topping. I think they react better and really take off when they have a good root system and a few nice sets of leaves before you top. So the short answer is yes. Top now or anytime you're ready. For clones I think it's best to wait until they have 7-9 nodes.
Topping gives you as many tops as you want. You can top a top as many times as you like. I personally use topping lst and supercropping. I think my veggers right now have close to 30 colas each. I top the 4th set of leaves, lst the 4 main branches a few times, top again, and lst those, then supercrop right before flower for an even canopy.