I sure as damn have never successfully deathbombed in a situation where I don't expect to be hit, for example when screwing up pacing while streaming, when I misread hitboxes, or in early stages while trying to grab items.
I think the deathbombing interval is 6 frames which equals 100 milliseconds. Reacting faster than that gets you disqualified in sports events, so chances are it's impossible to do without (subconsciously) anticipating the miss. The Internet says the average reaction time between a visual sensation and pressing a button is 215ms according to Google, and <150ms is extraordinary. You can give up dreams of being able to deathbomb on reflex to avoid dying with bombs, nobody else can do that either.
I guess in EoSD if you're playing for survival you might purposefully ram into a bullet whenever you need to bomb to decrease rank (affects bullet density and speed), but that would be a pretty fancy tactic. Just press X whenever you're not in control of the situation anymore.
"If you can't 1cc Easy, believe me, deathbombing alone won't get you to 1cc Hard." seconded. It's easier to no-miss no-bomb the whole game on Easy (esp. since it's only 5 stages) than to clear Hard using all resources. Compared to Easy it has some brutal attacks in the later stages.