If you have ICS you should have a virtual button or hardware key to display recent apps, and if you swipe them away it closes I think. The Android equivalent to the double-tap home button and touching and holding apps then hitting the red x on an iPhone.
Sometimes, in lieu of an exit option, hitting menu will bring a context menu, often with 'exit' being an option. Other times, hitting back will either silent exit or cause a 'hit back twice to exit' alert to display. Normally the back key only freezes apps so while running, they use no CPU or memory. I just hit back until the home screen displays if the app has no exit feature.
Samsung Galaxy Precedent (rooted, AreaRom 3.4 , sd-ext w/ 3.65GB internal space)
Kindle Fire (1st Gen, rooted, AOKP JB 4.1, Nova Launcher)
Samsung Galaxy SIII 4G LTE (rooted, Carbon ROM)