Sir:
I operated the 747 (SP, 100, 200, 300), the 400, and the L-1011. There were numerous times I and the F/O were not sure of clearance...most times probably clearance was ok. The key is...if you are not sure, STOP. The answer to pressure by ATC, is "NO!", and if more pressure is applied, "HELL NO!", with a little GFYourself implied. No Chief Pilot, or Flight Ops Manager, would dare question a Captain who said "I stopped because I was unsure of my wingtip clearance." I operated the same way in the US Air Force...if there was one UNFORGIVEN incident there, it was hitting a wingtip. Nobody EVER was punished for refusing to take a chance on clearance, but, if some poor chump 'took a chance', and hit, there was NO forgiveness. If there is ANY doubt whatever, STOP. There is NO downside for that (and, you are on the clock anyway). Huge downside for taking the slightest chance...your record, your career, etc. etc. No excuse allowed for poor vis, nighttime, etc etc. One safe answer..."STOP THE AIRCRAFT". Sam