I believe that the actuation of the fire bottles as part of pax evacuation drill is no longer required in the revised B737NG Boeing published checklist. Unless there is a engine fire of course?
In the B737 Classics and the -200, if you just pull the fire switches only in order to shut down the engines as part of the pax evacuation drill, the engines will continue to run for at least 20 seconds due to the position of the various fuel shut-off valves.
Actuation of the start levers to cut-off affords however immediate engine shut down. The Classic drill states (among other things) start levers-cutoff. Then order pax evacuation (engines running down by now) and finally fire switches - pull. If you reversed the order and pulled fire switches as first action and then ordered pax evacuation, it is possible that the escape slides would inflate and get blown away by the still idling engines especially if you failed to cut the start levers immediately after the pax evacuation order.