You don't need to, but it might be helpful if you are planning on vegging a lot longer. Another option is to just LST your plants by tying down the higher bud sites, allowing lower ones to catch up. Usually this means bending your main cola over. If you want to use a net, you could do some topping to increase the # of bud sites, then as they grow, you spread them around the net and tie them down, that way each bud site has it's own little space. That way you make the most use out of every square inch of your light footprint. Either works fine.