If we allow 'tipping' then people will start to expect to be tipped (if it gives reputation), and it will become another contentious point.
The system provides adequate means of thanking your answerers, there's no need to add additional layers of "thank you!" on top.
If you really appreciate their work, add a comment to that effect - trust me, they like being recognized.
Edit:
As a counterexample, I would love to tip both answers on this question I posted yesterday:
I'm trying to think of another way to reward multiple answers beyond the usual voting/accepting/commenting...