Plants have a genetically predetermined limit on how much they can produce, but you usually won't come near that limit indoors, so vegging by height, based on your room's height limit, is a good way to go. Some strains will stretch more than others in flower so you need to get a feel for that first before deciding which height to veg them to. Hope that helps a little.
I used to think that too, till I saw Heath pull 45 zips off one plant (critical mass aka big bud)/ indoors/ with 1800 watts.
Anybody doin the vertical thing?