Sue,
Good idea, but only partially correct. Throttling the supply can somewhat reduce the pressure in a flowing water system (while a faucet is open), but as soon as the trailer system becomes static (faucet closed-no flow), pressure will rise to match the pressure of the city or campground water supply (Pascal's Law).
Keeping the water supply valve partially shut will, however, help if a trailer pipe does burst due to over pressure... by reducing the water flow into the trailer.
Similar to the water pressure regulator on your house (assuming your house has a need for one), the simple pressure regulators used for trailers will maintain a lower preset pressure (usually 45 psi) even if the water system is static. Some city water systems provide water at pressures as high as 100 psi, or more!
Jim