[gearhounds]

Not at all foolish to avoid an avoidable exchange of gun shots. Be the best witness you can be instead, preferably from concealment (actual cover will likely be hard to find in a stop n' rob), then wait until his exit and try to watch his direction of travel, or get his getaway ride while dialing 911. Most likely, you will increase the risk to the cashier by attempting to intervening, unless of course deadly assault is already under way (which you wouldn't have been able to prevent anyway).