I leveled Ret and I had no issues getting groups for dungeons. Before like level 50, I don't recall ever being given a hard time about joining a group as dps. However, I did keep a set of healing gear on the side, and I would often join groups as a healer as well. While leveling up, you won't really notice much of a difference between healing as Holy and as Ret. You won't really have much crit or +healing regardless, so the benefits from the Holy talents aren't very pronounced. Honestly, even at level 60, being more than 20 points into Holy is somewhat overrated.