Originally Posted by
Private jet
As the captain of the aircraft YOU are responsible for the safe conduct of a flight, and you are perfectly entitled to question or deviate from any clearance or instruction from ATC if you believe the safety of the flight is or potentially will be comprimised. In that sense ATC is only an "advisory" service, the buck stops with the PIC, however you must be able to reasonably justify your actions afterwards!
Yup. Which starts with planning properly before take off. If you've done that and the subsequent perf means you're not going to make it, aviate and talk. It seems the knee jerk on here is to ignore poor airmanship.
Hat off to the OP for owning up to not planning before take off. The ILAFT is 'do the planning on the ground to avoid avoidable problems in the air'.