A full node provides several services to the network:
1•Broadcasts blocks (public service)
2•Broadcasts transactions (public/private service)
3•Increases privacy by hiding other node’s IPs
4•Increases network security by protecting it from global DoS.
5•Provides information filtering services to SPV nodes.
6•Provides historic blockchain and state information to new nodes.
With your tip idea you only encourages 6, and by increasing the number of nodes, also 3 and 4.
The services 1 and 2 cannot be encouraged by tips.
However, it's a good way to start.
I'll soon present a solution to encourage full nodes to store the blockchain based on Proof-of-Unique-Blockchain-Storage (PoUBS), a feature that RSK will add to incentivize Bitcoin and RSK full nodes. This solution encourages 6.