Don't make the mistake of thinking stalling speed rather
than angle of attack. The wing is generating lift while the airplane is
rolling on the runway even well below stall speed, simply because the
attitude is below stall angle. Flaps can add a lot of lift below stall
speed, as I could prove by getting a 172 off the ground with full flap
at around 40 MPH, in ground effect.
The 172's flaps reduce the stall speed mostly in the first 20
degrees, and the last 20 add mostly drag. There's only about 1 knot
stall difference between 20 and 40 degrees, so it's best to leave them
hanging out. If they're electric they retract too slowly to do much
good, and might actually hurt the stopping effort by removing drag and
contributing lift as they pass through 20 degrees. If it's an old 172
with manual flaps, dumping them on touchdown can help a lot.

Dan