Rings for hand towels are totally dependent on the fixture and the location. Obviously, you don't want it so low that the bottom of the towel is laying on the counter top. But beyond that, it's up to your esthetics. If mounting over a counter, try position the fixture so the top of the towel is 24-30" above the counter. If to the side, try 6-12" above the counter top.

Toilet paper holders are all over the place. My family is tall, so I prefer them higher and further out than typical. The best way to position your TP roll is to sit down with roll in hand, and hold it where it's most convenient (and out of the way - nothing worse than have the TP roll right where your knee wants to be :). If you're installing a recessed holder, determine the location of the wall studs first - you don't want to cut into the drywall only to find that there's a stud in the way.