Abra casts a spell. Naomi responds by putting Counterbalance triggered ability on the stack. Naomi responds to Counterbalance triggered ability by using Sensei's Divining Top. After Diving Top ability resolves, Naomi decides to let his Counterbalance triggered ability fizzle and not reveal a card.

I'm pretty sure thats that how it works but I would just like to verify. Is it still a “may reveal” triggered ability or does he have to reveal the card?

Current Oracle wording still includes “may reveal”.

Counterbalance
{U}{U}
Enchantment
Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same converted mana cost as the revealed card.