Feed Veg nutes all the way until the stretch stops (usually between week 5-7) even if you see bud formation if you want to get the most out of your plant. Switching to flower nutrients early will definitely hurt your yield. If you want a small plant, or NEED your plant to finish faster than go ahead with flower nutrients when you start to see buds.
Most likely running Bloom nutes on young auto's will cause overall unhealthy, deficient plants prone to wilting easily.
Auto's are VERY picky about what they want, and when they want it. They are also very nutrient sensitive.
With a regular plant some people will start the seeds at 1.0 EC and run that all the way through flowering.
With auto's that is not possible. You have to start nutrient solution LOW and SLOWLY build up.
Check the links in my sig if you want to check out some pretty killer autoflower grows.
Peace.
Taviddude