Autostart of an LPAR should not result in any problems as long as every resource the LPAR needs is available at boottime. As long as you use dedicated disk and adapter devices from your p4/p5whatever this should almost never be a problem. However, I have seen this being a problem with VIO and SAN disk devices that where not accessible in time. I.e. make sure that you set up your environment in a way that there are no potential obstacles in the LPAR's boot process.