Is it OK to greet ATC?
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.