As I understand it, if your'e plugging individual appliances/tools into the generator you don't need to ground it. If you're connecting it structure to power everything inside, then you do need to ground the generator.
From:
https://www.osha.gov/sites/default/files/publications/grounding_port_generator.pdfGrounding Requirements for Portable and Vehicle-mounted Generators Under the following conditions, OSHA directs (29 CFR 1926.404(f)(3)(i)) that the frame of a portable generator need not be grounded (connected to earth) and that the frame may serve as the ground (in place of the earth):
• The generator supplies only equipment mounted on the generator and/or cord- and plug-connected equipment through receptacles mounted on the generator, § 1926.404(f)(3)(i)(A), and
• The noncurrent-carrying metal parts of equipment (such as the fuel tank, the inter- nal combustion engine, and the generator’s housing) are bonded to the generator frame, and the equipment grounding con- ductor terminals (of the power receptacles that are a part of [mounted on] the genera- tor) are bonded to the generator frame,
§ 1926.404(f)(3)(i)(B).
Thus, rather than connect to a grounding electrode system, such as a driven ground rod, the generator’s frame replaces the grounding electrode.
If these conditions do not exist, then a grounding electrode, such as a ground rod, is required.
If the portable generator is providing electric power to a structure by connection via a transfer switch to a structure (home, office, shop, trailer, or similar) it must be connected to a grounding electrode system, such as a driven ground rod. The transfer switch must be approved for the use and installed in accordance with the manufacturer’s installa- tion instructions by a qualified electrician Grounding requirements for generators con- nected via transfer switches are covered by Article 250 of the National Electrical Code (NEC).