I often don't use it (never for the initial traversing of Dark Cave, Hoenn, for example)
Some places are really hard to traverse without it, though.
Have any of you ever tried getting though Mt.Silver without Flash?
I have managed, more than once, but it required very carefully counting my steps to match to the map, and listening closely for the bumps. It is not an easy thing.
If the cave is really difficult to navigate I will use Flash, but if I have a reason to not I can go without.