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depends on the site, some you need the active, some you need the posting, oh on steemit you can log with your posting, the idea is to keep the active secure, and the owner basically you need to know the risk, you can get locked out of your account if someone gets that and changes your keys, then you have to contact your recovery account and it's great there is such an option, but in short be careful with the owner key, refrain from using it and don't keep in on your pc desktop :D

so yeah, now I'm thinking I could have responded with "try it out" :D keep your mind open, learn mate, just learn, try, break, fix and try again :P in this case the worst that can happen if you try is you have to login again because you don't have your active key or posting key, plus things might change so you have to be on your toes.