There is some research using valtrex to prevent cmv, hhv6 after organ transplants. So it's worth a shot. But my usual speal is that famvir going by peoples experiences here is that famvir has better effects on cmv and hhv6.
After 6-12months of valcyte it wasn't uncommon for people to stay on valtrex or famvir to prevent or slow down viral reactivation after a valcyte course.
This was about 15yrs ago if you search some of the old threads on here.