Starting the chromo while starting the engines is a TECHNIQUE, not an SOP. The rational is that if you have a start abort and subsequently have to complete a dry crank and or manual start, it is the only way of monitoring the amount of time you have had the Start Valve Open with respect to the Engine Starter Limitations.
Other people can argue the validity of this technique.
After start, after a shut down period greater than 2hrs, to avoid thermal shock, the pilot should operate the engine at or near idle for at least 5min before advancing the thrust levers to high power (above 40% N1). That is SOP.
After start it is for that reason.