Yes. You do.
Even if you're talking about pre-flowers..it's not accurate and therefore not a valid form of determining sex.
Here.. pick a result and read it.
http://www.google.ca/search?sourcei..._enCA299CA299&q=sexing+marijuana+plants+early
My favorite result though explained it nicely from a RIU member..
https://www.rollitup.org/indoor-growing/17987-early-sexing.html
"Preflowers, as opposed to full blown flowers, generally appear after the fourth week of
vegetative growth from seed. Check carefully above the fourth node. Please note that preflowers are very small and and almost impossible to differentiate without magnification. A photographer's 10x loupe is handy indeed when examining preflowers.
Frequently, the female preflowers do not show pistils until well after the preflowers have emerged. Thus, don't yank a plant because it has no pistils. Pistillate preflowers are located at the node between the stipule and emerging branch.
Also, some female preflowers never produce pistils. A female preflower without pistils is difficult to distinguish from a male preflower. Thus, hermaphodite issues should not be resolved by the appearance of preflowers, without pistils, on a plant otherwise believed to be a female."
Hmm.. seems to me that if it's unreliable as to outcome.. it's not a method...BUT forcing into flower IS the trusted way to determine sex with accuracy.