Sadly staying offline won't keep your work 'safe'
I've seen people take a snapshot of some painting or whatnot, or a scan if it's in a magazine, then 'fix it up', slap it onto the net and there you go. Art thieves will always be around and always take credit from what they see and can take and change.
So...if you try hard to keep it offline it will make it harder for people to snatch, but some still will if they really want to though unless you only show it to friends and family, but then no one gets to see your work except the few that are close to you.
I know more than a few pros that post here, my friend for example
so it's just ignorant people that say dA is just an art dump site when we have a lot of people that are making their living off their art yet they post here.
Personally, in the end, I think it's better to keep posting anywhere you can to get whatever feedback you can and get your name out and around but also knowing that if someone is going to steal your work, they will regardless of it being on the net or not.
~Dale