I always do it if I am trying a new strain. I will harvest at perhaps 5% amber, 25% amber and 50% amber for example, dependant on strain. That way I know in the future which effect I like best and I can time the flushing etc to match this..
It doesn't really matter where you snip from at this point. Just try to go for a couple of clearly different stages
I was in no doubt that those lights will produce as I have seen several great ones. It literally is just the cost issue that makes it unviable for a lot of people. particularly if they step up to 600, 1000 watts. I'm sure we will have LED grows eventually here, once customers can follow suit so to speak. Mind you I would have to find a more manly colour haha