Perfectly viable to level as Holy, particularly at earlier levels. At 89/90 you will start to notice mana troubles if you're bouncing consecutively from mob to mob but this is only an issue on the most grindy quests. The spec you choose is largely irrelevant. None of the talents have a massive effect on Holy dps, other than Sanctified Wrath, which is by far the best option in that tier for Holy. At 90 you'll want Light's Hammer so you can actually do some AoE damage.
They glyphs are what is important. Pick up the Holy Shock glyph, Harsh Words and Word of Glory. Make good use of Divine Favour and Avenging Wrath as often as possible as they speed things up considerably.
You should have no problems healing dungeons with the above dps levelling spec but there's always dual spec for a full healing focussed spec anyway.