Yes it is safe. Most people who get hacked on Steam is due to thier own negligence.
For example, someone wants to be your friend and you give them your password to your account, and something bad happens. Why would you do that???
Or, you get a email or a file from someone to "unlock" a feature, or a mod which is actually a virus. Why?? Make sure its a trusted source. If you are unsure, you're better off without.
You get a email that your account will be closed unless you provide info, such as account name password etc.
((FYI Steam NEVER EVER asks you for a password. They will also NEVER contact you in such a way out of the blue. You only recieve support response from thier website on a ticket you open. Never through a email reply.))
That type of stuff.
Steam is safe provided you:
1) Never give your password and account name to anyone.
2) Do not open mails, click on links, or install executable files to your PC from a unknown source.
3) reply to emails asking for your account information for "verification purposes" often in broken english.
4) use SteamGuard
Pretty standard security precautions for anything dealing with the world wide web though. As with anything, theres always someone trying to hack into or steal something thats not thiers. Just be smart about who you trust and what you download/install to your PC.