As far as hanging height is concerned, it's totally subjective. You're looking at them, so put them at a height that's good for you. It all depends on the size of what you're hanging, the size of the room in which your hanging it, the available light, position of doors and windows, etc, etc. No rules, just do it. Just be sure to use fixings that are sensible for the weight of whatever you're hanging.
Get your other half engaged in the process by asking them what height they should be positioned.....
You mention no rules - I suggest one, although I've not yet been a victim.
DO NOT!!!! REPEAT DO NOT!!! Be driving nails/screws/screwdrivers/drill bits into locations that are directly inline with electrical fittings, or where pipes may lie hidden....use a tester to find such things, before you even THINK of getting any other tool out. I know 99% of people probably don't, as the 1% aren't around to recall their experience!
Also make sure the fixing of any wall attachment is secure. I'm RUBBISH at DIY, and fitted a glass shelf alongside the bath into plasterboard fitting, with special rawlplugs especially designed for the job. That didn't last long. A friend of mine had an expensive Hifi on the table, and an equally expensive picture hanging above it, until one night, the picture fell, killing both. I 'used' to lie on the floor at my parents house, before I was booted out, one day the budgie flew over to the ornate mirror above the fireplace, in the '70's style of a sun, with sharp jagged edges...I was snoozing, next thing, hear a crack, do a Jack Bauer roll, and the mirror splinters into a million pieces where I was just lieing.
And even worse that close death...I suppose...maybe joint top, nicely asleep, when I hear an almighty crash downstairs, thinking the house has been ramraided or something, at 4am, and bleary eyed, I go down with half a snooker cue in hand, naked to investigate, with my partner clinging to my shoulder. There is thick goo on the floor...there must be a murder here? Walk further, the crunch of broken glass under bare foot, the pain isn't noticable, what demons have been here tonight?
She flicks on the light, and it's only the food store cupboard that has fallen off the wall! All jams, and syrup and glass produce are spread across the floor. Coffee, is now intermingled with teabags, and maple syrup, broken glass and tins have exploded.
So make sure those fixing are good!
Traditionally a home would have a picture rail, so depending on the style of your house, maybe reinstall these if they are in the style of the property? I'm sure someone would have the dimensions for a picture rail? Or just hang the pictures as you see fit?
But (in evil tongue) - heed my warnings!

good luck!