It depends on the look you're going for, and how much you want to be able to see at night in darker places (not a lot of street lights, etc.). I've currently got 35% all the way around, and on other cars I've had 5%. Personally I don't think 5% looks right on the Sonata, and you can forget about seeing out the back window at night when you're trying to backup. Same goes for the side windows if you're making a turn at night, like turning onto another street in a dim neighborhood or similar. I used to have to roll my window down making a turn in our neighborhood since we are street light free !
Even though I currently have 35% if I had it to do over again I would go with 20%. The 35 is just a tad too light, although the wife loves it. But, it is very easy to see out of at night.
Here's a pic in my Garage of Gracie with 35%:
35% Tint