No issues running a properly sized extension cord to charge. Cords come in all different lengths and gauges, so Tesla just opts out of trying to provide any guidance, lest someone melt their 100 ft 20awg 5-15 cord. I use a homemade 25ft 12awg cord with 6-20 ends on it to charge at a family member's house for over 20 hours straight, and I used an 8awg 14-50 extension cord to charge my Lightning for 6 weeks from the 14-50 in my garage.
That said, I don't think I'd make an extension cord part of a permanent installation plan, but there's nothing wrong with using them as they were intended.