A straight cue is not critical. I prefer crooked for the open break at times, as mentioned in “Powerful Open Breaks” in Picture Yourself Shooting Pool. One gains perspective with a pre-bent cue. The student feels a twisting flex like the pro feels on a powerful stroke with a straight stick.
Another time, I spied on cheaters bending the break cue of their opponent to better gamble at “3-Ball”. The “thwarted player” made all three balls on the break with the bent stick, several times!
My pal and certified instructor Donny Lutz reminds me, however, that a cue that starts its life "nice and straight" will likely remain straight if cared for properly. So when buying a cue for myself or to re-sell, I always want it to be straight. When I buy sticks on eBay, I always ask the seller, “Do the butt and shaft both roll straight (on a pool table), both together and apart?”
I’ve debated crooked cues for years. I remember one 1992 session playing David Uwate (ACUI Champion that year). We ran 9-Ball racks with the most warped cues players could scrounge.