Just to point out one thing regarding this. If you want to retopo models that are scanned by you that's ok, but if you plan to retopo other peoples models (with this i mean, other peoples work, charachters ripped from games etc.) that can and will get you into big trouble. Because as naive this question is it isn't because now everyone who have even slightest amount of 3d knowladge can use those tutorials and advices do fast retopo of models that will end up on sale here and other marketplaces. In my opinion this kind of discussion should be talked in private.