This article on Houzz.com talks about the fun and flexibility of leaning artwork. We'd like to add our a few words of caution: when leaning art and (especially) mirrors, keep in mind that they can slip, fall, break, damage, and injure (sigh!) unless properly secured to the wall they lean on. Large items should never be leaned as a shortcut from their secure hanging.
If after reading this article you consider leaning items throughout your house, please feel free to call us for some free advice.
Leaning art can look temporary, unfinished, and somewhat unsettling... On another hand, we like the ease of switching and rearranging pictures! It never hurts to try.