It depends a lot on the agency and on the accident scene. Some departments here transport to a preplanned LZ almost all the time, and some prefer to set up an ad hoc LZ at the scene. The scene environment makes a big difference, and some are more conducive to landing a helicopter than others. As a pilot, I don't care much, I'll do whatever the ground unit thinks is best, but landing at a prepared LZ does present fewer challenges than an unprepared LZ, both to the pilot and to the ground unit. Just let us know where you want us and we'll be there, but please make an effort to survey the LZ for obstacles and wires. Trees, etc aren't a big issue, but wires and stakes, stumps, etc can be, and I would like to be warned about these, even though I never ever fully trust the ground unit to find all of them. I know they try, but I also know how easily they can be missed. As long as you make the effort, I won't complain.