I think its more important to keep your ereader in a sturdy case while carrying it in your bag rather than while reading it on the bus.
Seriously, most screen fractures seem to occur due to twisting or flexing of the screens, and the crack often appears at a later date, after a slight bump (like when you place it down on a table) and people then think "what's happened, this thing has broken and I never dropped it?". So when you carry it in bag or pocket, make sure its in a sturdy case that will protect the screen, and don't let any other objects lie against the screen while its in your bag.