Yeah... It doesn't really matter, but 6 is still more stable and more compatible/supported. It's still better to compile against 6.
Compile with JDK 6, run with JRE 7. If you compile with JDK 7, people with JDK <= 6 won't be able to run it. You can also run with JRE6, but JRE7 includes the new garbage collector, which should increase the performance.