I recall that users on bitcointalk and other crypto discussion boards post wallet addresses that people can send tips to. I haven't looked at many profiles here so maybe users are already posting them here as well. Are there any rules or requirements for posting ether, bitcoin, or other addresses to receive tips? If not, should there be? It would be annoying if users posted addresses in every answer they gave - on the other hand it's not a problem yet so no need to add bureaucracy at this point.
And privileges are important because this site is run by you, the community. High reputation enables you access to moderator tools, protecting questions, deleting posts and finally even access to site analytics.
However, you should be allowed to post a tipping address in your profile. As long as you don't post it along with content or advertise for tipping, you should be fine with that.
In my opinion we shouldn't allow posting of addresses in answers; it seems that people could post a tipping address on their profile if they desire and could get tips that way.
I think a free tipping / donating is important in the crypto space as so much is open source and so many people create amazing things... And get not much in return. However, I agree that including a tip address as part of an answer / asking for tips for your answer is taking it a step too far.
However, I do try to include the donation addresses of the developer (if they have it posted in the github or whatever) when my answer includes a library or service or site that solves the OPs problem (not my own, though).
a tip contract would be a good use case : send eth to the contract, precising tip address, tip amount, stackexchange question id
At Oraclize we have designed this PoC DApp for depositing bounties over StackExchange questions. Right now it only works in testnet, but it can be developed in a reasonable time to a fully fledge DApp on Ethereum main-net, if the community like the concept. You can read our blogpost here and take part in the reddit discussion here. Feedbacks are REALLY welcome! :)