I do the entire dash and seats! Just don't get it on the windows since it's a bitch to remove.
This little trick was taught to me by a chemical engineer who lived in AZ. And well plastic parts in AZ cars just dry rot fast due to the UV light and extreme heat. When I asked him how his car looked good and had no cracks he told me about the "Black Magic" trick.
I did it on my VW Jetta and it was squeak free from the day i bought it till the day I sold it.