http://www.tfl.gov.uk/dlr/tickets/oyster_faqs.shtml#9 says:

"How do I use Oyster Pre Pay at Bank station?

If you interchange with the DLR at Bank you need to touch your Oyster
card flat on the yellow reader again on the DLR concourse. If you do
not touch in and out at Bank's DLR concourse you may be charged more
than you need. This is not necessary at other interchange stations."

Which suggests that you *do* need to touch out at the DLR concourse,
though it isn't clear whether you still need to do it if you intend to
exit Bank station though the ticket gates immediately afterwards
(obviously you need to do it if you are using the lift to King William
Street).

I've never done this, I just touch the reader at my end destination,
and have always been charged correctly. Is this only necessary since
the £4 charge was implimented? Surely touching on the reader will end
your current journey, and when you touch out at the DLR station you get
Off at, it will start a new journey?