Rows 2, 3, and 4 of NESC Table 232-1 are based on a vehicle height of 14 feet (per Appendix A). If I know vehicles with a height greater than 14 feet may be operated under the line, do I need to increase the clearance?

Yes. See both Appendix A and Footnote 26 of NESC Table 232-1 for clearances where vehicles taller than 14.0 feet may be encountered. Clearances should be increased to account for farm equipment, dump trucks, or other vehicles exceeding 14.0 feet in height.