Aftermarket stereos are connected to ignition therefore will not drain your battery.
It's the OEM ones that drain your battery because even if the ignition is off you can still switch the stereo on unlike the aftermarket ones like pioneer, alpine etc.
So if your upgrading to an rcd310, 510 etc you will get drain issues.
Aftermarket stereos will have a seperate feed to your ignition and therefor will not drain your battery.
Hope this helps.