It is a valid question to students, as I'm at the stage where I practice them solo (could be the topic of a discussion in its own right, I know). Anyway, I was never much good at PFL's until I'd done a couple solo and this really boosted my confidence. As to the height - always try to go below 500ft to make it a bit more realistic but not usually by much. It's surprising how different the target field looks from 300ft or so - which is good for the practice but bad for the nerves.
As a side point, how are you to be sure you're not within 500ft of person or animal until you're well below that height? A couple of times on climbout from 200ft practices (with instructor) we've flown over people we hadn't previously seen (picknicking in the corner of a field and hiking behind a hedge) - makes me wonder if it's actually possible to stay 500ft from anything with any level of certainty.