Soft tires are a poor choice for Turkey, as it has a "Very High" tire wear rating. For tracks with tire wear ratings of "High" or greater, its best to stick with hards. Note that you can find tire wear info, along with other stuff, in each individual tracks information page (click the race in the schedule).
There are cases where softs can work on high tire wear tracks. They can work very well as a quali/first stint tire on these tracks. Most people will be qualifying/starting on hards, so by using softs you can not only easily out qualify those on hards, but you will be quicker off the start as well. You'll have to shorten your fist stint and then run longer stints afterwards on hards in order to compensate for the shorter life of the softs, but the track position you gain more than makes up for it. Using this strategy in a 50% race in China, I managed to scrape out 10th place. This is while only being level 2 up against around 26 other cars/13 teams, with almost all of them having more races under their belt and higher scouting/development levels.
Softs can also work well if the track is very cold, since hards take forever to heat up in cold temperatures whereas softs heat up fairly quickly.
Now I'm only 6 races into my career, so I don't know what strategies work best at each track. I picked up most of this info from the iGP forums, from posters who have been playing this game for quite a long time. From the advice that I've taken from there, most of it has been pretty good and has really helped me out so far in my career.