Imagine a scenario where you have already trained your Attack and Strength and or Defence to, say, all level 60. You have an HP level of probably <60 (given you haven't Maged/Ranged). At this point you go and do Waterfall Quest. Is your lack of HP XP compared to your extra 12.5k in Attack and Strength a big deal?
25K XP in atk/str/def = 8312.5 HP XP
(1.33xp in HP for 1 damage
4xp in ATK/STR/DEF for 1 damage
1.33 / 4 = 0.3325
25000 * 0.3325 = 8312.5)
No, it wouldn't be a big deal at that point.
Basically, for completing Waterfall Quest your HP has 8.3K XP less than it would have if you had trained those levels manually. The higher your HP level the less it matters. So, if you're going to be training your combat for a while it doesn't really matter. If you're going to be staking right after Waterfall Quest on a fresh account then you should get those levels manually, or get one of or a combination of atk/str/def to a total XP where that total XP multiplied by 0.3325 is significantly more than 8312.5.
Don't forget that you still have to get 12.5k XP in Defence to catch up with your Attack and Strength, so there's some HP levels to be earned there.