Depends on the aircraft, really, and the operation. I've seen a lot of pilots attempt to rush though the proceedures by flow, and catch up later. I've seen pilots completing the after start checklist in the climb, and even the before start checklist in the climb.
I fly a definite flow-airplane, and there's little or no stopping coming or going. I ran the flow this year on a departure for a going fire, one side of the cockpit to the other. I omitted the printed checklist, even though I always do a verbal challenge and response when alone in the cockpit or in a crewed cockpit. As I powered up and began the roll, I realized the controls felt stiff, and I found the drop-down internal control lock was in place. Proceedure in that airplane has been to install the lock upon rolling out and slow enough to do so, and remove it prior to locking the tailwheel for takeoff...the control surfaces need the protection.
The result was a takeoff roll with the controls locked. I was able to pop the lock safely and continue (as an abort in that aircraft is very unwise when loaded)...but that's the price of stupidly failing to execute the checklist on my part, at the phase of flight when one should...when I elected to unwisely rely upon flows. Flows that were executed twice; once after landing, once before takeoff (no shutdown inbetween; hot loading, hot refueling).
Checklists are given in phase for a reason. Flow, and back it up with the checklist, but executing the checklist when it should be executed instead of waiting until the aircraft is completely over that phase of flight is unwarranted, in my opinion.
I got tied up on a drop recently which was a formation run down a long canyon into the sun with high terrain close on all sides, to hit a target that was hidden in a surprise turn at the last minute on the bottom. Again, I concentrated on the flow and the run-in, and there was a lot going on. At the bottom of the run, I applied power and the temperature came up, indicating gas generator operation, but no torque was available. No power; I'd experienced an engine failure. I glanced down to check that the fuel boost and ignitors were on, and they were not...they would have been had I executed the checklist prior to joining up with the other tanker on the run.
I turned them on quickly, and fortunately it wasn't lack of fire or fuel that caused my engine failure. I had a catastrauphic failure with a failed bearing seal that ported all my oil overboard...but imagine if it had been a fuel flow issue or flame issue...my failure to execute the checklist, despite mitigating circumstances, would have meant the impossibility of a relight were it required at an extremely critical time. The flow didn't catch it; flow backed up by the checklist, verbally, would have.
I preach and teach risk elimination; had I executed the checklist, I would have eliminated that risk. I'm a firm believer in use of the checklist when it's called for in it's appropriate phase of flight or ground ops...even if that means continuing the checklist during the taxi. My own stupid mistakes and behavior keep reinforcing this; with luck I'll learn the lesson before it hurts me one day. I hope the same for everyone else.
