What are the common private pilot practices for helicopter off-airport landing in US?
There are minimum altitudes for cruise flight but altitude may be reduced below these minimums as needed for approach to landing. I don't know of any FAA regulation that requires landing at a designated airport(or heliport)

Title 14(aerospace) of the federal code of regulations part 91.119 (part 91 is general flight rules for all aircraft) simple says that; 
"Except when necessary for takeoff and landing, no person may operate an aircraft below the following altitudes:"

A: Anywhere. At an altitude as to create an "undue hazard" to people and property on the ground in the event the engine fails. (Will you hit a building or dense crowd, or will you have time to aim for an open spot?)  

B: Over any congested area of a city or open air assembly of people, below an altitude of 1000feet above the highest obstacle within 2000 foot horizontal radius from the aircraft.

C: over other than congested areas, no less than 500 feet above the surface, except for over water or sparsely populated areas. In those areas the aircraft may not be operated within 500 feet in any direction from buildings or boats. So no actual minimum altitude limit if you are over a field 500feet to the side of a barn.

D:Helicopters, powered parachutes, and weight-shift controlled aircraft(hang gliders and ultralights), if conducted without hazard to person or property o the surface may be operated at less than the minimums prescribed in B and C, provided the operator of a helicopter complies with any special routes or altitudes specifically prescribed by the FAA.(Generally special provisions are only found in high air traffic areas near major airports or over high security areas.)

These are paraphrased slightly for clarification and because I type how I likes to type, but the actual wording of the law is very close.

There may also be some local laws about noise limits if operations are frequent but this is not limited to aircraft.