What you need is all relative to what/where/what time you ride it.

If you plan to park your carbon fiber roadie in a dark alley over the weekend you'll need a lot more than myself who parks in an area surrounded with surveillance cameras during the daytime.

If you don't feel too safe, look at what others are riding and mimic that.

Just because your bike may LOOK like a beater, does not mean that it has to perform like one. Thieves will most likely look at the frame, not at your dura ace derailer