There are numerous threads about the effects and no-effects of the star system.

I've personally posted somewhere the difference of 0 stars up to 7 stars. It's really not much.
I have the results of 7 stars to 13 stars, with the difference between each step, but I haven't posted that yet.

Best tip: Use tickets for medals. Can never go wrong with that.
Even if you fail dozens of times, its ok, since tickets are basically free.