Always dog-ear. Sure, it's a bit paranoid, but I'm always afraid the bookmark will fall out somehow =_= it has before, so I cannot trust it... Plus, if the dog-ear unfolds, you can still find your place because of the crease :) HOORAY FOR DOG-EARS!! :D
I dogear them. I also sit drinks on them... and pile things on them in my packpack that bends their covers... and leave them in the floor for puppies to maul and/or widdle on.
It's tough love. XD
I always use a bookmark, even if it's just a scrap of paper, not because I don't want to harm it (heck, I write in my books all the time), but because I've just gotten into the habit of using a bookmark. I have a special one that "highlights" the last line you were reading on, because I have a really bad memory for where I stop reading, unless it's at the end of a chapter :P
I'm a bit OCD when it comes to how people "bookmark" their books, I usually will rip up a peice of paper and stick it in their book if they dogear it or leave it sitting open.