Glad to see people out here have interests in cephalopods.
It's always a good idea to button your tank up, but believe it or not I don't on any of my tanks. Depending on the species you acquire you may need too how ever. There has been recent discussions in the ceph area what species are prone to "escape" and those that don't. Conditions of the water and tank set up "do I have a place to hide?" is also a point of stress that can cause escape.
Certain species are known to escape and will try most of the time and other seem to have no interest in leaving their home. O.vulgaris,O. briarieus,O. joubini are species that will leave the tank given the chance. Other species like O.bimaculatus, O. bimaculoides seem to have no urge in leaving there home. I had a rare O. macropus species that hated to have even the slightest bit of arm out of the water and would only come to the top of the tank to feed.
Most people have no idea what species there getting when they order off the web or in the pet store. There are not may people that can identify species unless they have studied them for years. I have orderd a bimac off the web and received a rare macropus. A friend ordered a O.vulgaris from the Atlantic and received a bimac, a ceph from the pacific. You just never know some times.
Some recommend using a all mesh top made out of fiberglass window screen to allow the tank to breath, and swear by it for keeping octos in. I my self have not had escape so far, but I know what species I have and what there prone to do. A friend has almost a dozen cephs and has not had a escape as well. Most of the cephs were bimacs. Until you feel comfortable you can recognize what you have its a good idea to keep the lid on things, no matter how you do it.
My friend Jack at fishsupply.com is your best bet for a bimac, he knows how to identify a bimac and will send you a nice one. Make sure you tell him you want a bimaculatus. Just tell him the octopus guy sent you.