First off it takes a time period of 4 to 8 weeks vegetative growth for a plant to reach maturity where it will show it's sex, in some sativa's it may take longer. Plants will not show their sex until they reach this level of maturity. An immature plant will not show sex because initial calyx development is not photoperiod-related.
Problems caused by starting too early.
Stress-related Sex Problems (Hermaphrodites)
also
Abnormal Bud Growth
Abnormal bud growth is a side effect of the hermaphrodite condition. Because the plant produces male pollen sacks with female flowers you may notice that the bud looks different. Also, the quantity of female bud produced is decreased because of pollination.
Force flowering simply acts by stressing the plant into a crisis condition.
You will get the best out of your plant by waiting until it starts pre-flowering before switching to 12/12.
Will it effect the quality ? It effects the overall size of the plant from the stress so I would say yes the plant won't have it's full strength potential as if it was grown stress free environment.
Bottom line is you aren't gaining anything by force flowering, but you are adding the potential for a whole lot of other problems, not to mention the added stress. Really I see no point in even considering it.