Nope, you won't have trouble healing unless the tank is under-leveled, or pulls too much.
At lower levels, you really don't get much of a bonus to healing from Holy or Disc talents; the difference is not noticeable until the late 30s.
Even so, you can safely shadow-heal instances up until Blackrock Depths or so. After that you will want to be Holy. Even in BRD, you could probably get away with shadow-healing most of the instance, but Emp will be a challenge with how obscenely hard he hits.