In terms of efficiency and maximizing your stash, you should take smaller hits and be sure no visible vapour comes out. Exhaling until you see a wisp of vapour and then inhaling new air and holding your breath ensures more oxigen gets into the lungs to assimilate the remainder cannabinoids. Then there is the psychological factor from our pre-vaporizing days, where you exhaled a big lungfull. Plus, don't forget that big hits are usually more felt on the throat and give that rush of having your lungs filled to the brim with vapour and then letting it fog the room. On the other hand, there is the visual aid that clouds give you, allowing you to have a way to evaluate the thickness of the hit and the dosing you are receiving, like T-Dub said. You can use a bubbler for a similar, although not exactly the same, effect. And finally, a predominantely convection vaporizer gives more vapour output than a predominantely conduction vaporizer, and a vaporizer that can maintain a steady temperature for longer draw times is able to produce bigger hits. So, the ability to produce big clouds is also a good measure of the quality of the experience you will get with a specific vaporizer. Now, how you choose to vaporize it is entirely up to you. Sometimes i hold it in, sometimes i don't. But, if i had unlimited quantities of herbs, i would produce big clouds most of the time though. It is just more fun!