Is it OK to greet ATC? My instructor told me that courtesies are forbidden in radio communication, however, we usually start conversation as "Turany Ground, OK-ABC, good morning, (... request)". 

I find it natural. Is that acceptable violation of the rules, or could it cause some troubles?
It is not part of the standard phraseology by any means, but it is fairly common, as is a very quick pleasantry when changing frequency. 


  [station name], Good morning, G-ABCD, [request]


and 


  G-ABCD, changing to [other station] thanks for your service.


The thing to remember is to not choke up a busy station with overuse of non-standard comms, and therefore to remember when it might be inappropriate to stray from the standards.