You would figure ATC would be on top of anything that looks like assuming when it comes to runway safety. If I encountered a situation where a controller gave me a hard time about timeliness after I followed their directions to a "T", promptly, I would definitely follow up on that. Runway safety should never be a guessing game.
Personally, if I was flying with someone who jumped the gun with an excuse like the one mentioned in the OP, I would start with a tactful "intervention". Wording my objection to behavior in the form of a question has had the best results, although it doesn't always work when the other is convinced they are right. "Don't you think we're jumping the gun by taxiing across the line before the controller clears us?" If that doesn't work, I look up the regs and reference them in tactful way. "I'm confused (in a non-sarcastic tone), my interpretation of 91.123 is that we have to comply with ATC clearances. I understand that an impending clearance is expected shortly, but don't you think we're still on hold short clearance until they officially give us a new one?" "Also, in the AIM 4-4-10 it says: When air traffic clearance has been obtained under either visual or instrument flight rules, the pilot-in-command of the aircraft shall not deviate from the provisions thereof unless an amended clearance is obtained...ATC in certain situations, will include the word "IMMEDIATELLY"...
Right there in the AIM, they have a plan for when things need to move faster. If none of that stuff works and I feel that the other crewmember is out of their mind and needs word from the rest of the villiage, I will contact Professional Standards. I know there are some folks that will object and say this isn't worth the effort, but I don't want to get killed or violated over things like this. All it takes is one controller to screw up and at best, they are going to cover their butt, pull the tapes and say "look, they crossed the line and I never did clear them." No thanks!