I'm on clay and get 2" wide cracks in summer which dissapear in the winter. I wouldn't put anything direct onto the soil, I'd put some sort of subbase down first.

http://www.pavingexpert.com/subbase.htm

http://www.pavingexpert.com/faq_subbase.htm#reqd

I reckon if the ground is going to freeze and lift slabs it will probably do it every winter regardless of when it's done. Obviously if the ground is already frozen you shouldn't lay paving..

Scroll down..

http://www.pavingexpert.com/faq_weather01.htm

That site is worth bookmarking.