A plant can show pre flowers as early as 3 weeks of veg so no it's not an indication of the flowering phase, this is the same for Autos or Photos.
Once you see pistils developing, you've entered the flower/transition phase.
During this phase the plant will stretch, you will want to continue with your veg feeding and keep light intensity low.
After the stretch, start raising light intensity, start lowering veg nutes, and start raising bloom nutes.