On the shorter 777 versions like -200 and 777F Vref30 is limited at light weights for controllability reasons (VMCL). Might be a sensible to use flaps 25 in normal ops on these tails if only light weight as the Vref30 and Vref25 will be very close or even the same and Flaps 30 with these speeds.
However I have no real world experience with this effect as we only operate the -300ER.