Yes, you can tap A for shot. No one with a good score does that, though (at least not regularly). Everyone I've seen play (including myself) holds C for shot and presses A for laser when needed. Many times in stages and on bosses it's beneficial to TAP C (shot), as this stops your lasers firing (reducing the damage you deal) but still increases your chaining bar in Maniac mode. That's why I wish they gave an option for a rapid C shot in the button config. Pretty big omission in this port, if you ask me.
When I play Original mode, I press A for laser when needed, and C for shot when needed, but whenever possible I don't hold C while I press A as this can reduce the number of gems you get. In other words, I hold A, let it off, absorb gems, and repress A.