Being a rather new but still average-to-terribad player (depending on my mood, the time of the day, the weather, the number of times I had to ragequit and restart due to stupid deaths in the first three stages, whether the planets are aligned or not, etc), I have found that the only way I can deathbomb consistently is when I know I'm going to get hit. Otherwise, it's just my face going "WTF JUST KILLED ME" and not hitting the button at all, or hitting it way off the time window.
This, unfortunately, happens to me a lot during the early stages since I know I can pretty much perfect stages 1-3 in every game (bar from SA) consistently if I concentrate enough, which leads me to being lazy and complacent, thus suffering from constant stupid clipdeaths. Otherwise, when I'm reaching the more challenging stages 4-6, my deathbombing skills suddenly skyrocket since I'm expecting to get hit all the time.
Also, if you're playing PCB without the vsync patch, deathbombing was nigh impossible, at least for me. Not only PCB became actually playable for me when I tried it, but I realized I could, actually, deathbomb in that game (PCB was the second game I played, after IN whose deathbomb windows is obvious enough).
PS: also, fast patterns are noticeably easier to deathbomb than slow patterns. Slow patterns usually mean clipdeath for me if I screw up since OMG THIS IS EASY I CAN DO IT