In our B737 NG FCOM Vol 1 it is stated in the chapter "Limitations" under "Low Weather Minima Operations" that:
The airplane is approved for the following types of ILS approach and landing:
(.......some text omitted)
DH below 100 feet but not less than 50 feet (Category IIIa) (Flaps 30 and 40).
- Dual channel automatic approach and landing.
In other words we may use flaps 30 or 40 at our discretion.