I think that patrons should tip 15% for okay service after taxes, 20% for good service after taxes, and 10% for poor service after taxes. Unless it's a higher end restaurant, then you should adjust by adding 5% to each of these guideline. The only time I've ever left no tip for a server is when I was actually offended by their service.
The reason that I insist on tipping after taxes is because servers have to pay tip outs to the restaurant and suppost staff based on the post-taxed amount of a bill. So, if a patron is under-tipping, the server may be giving away their total tip and thus receive nothing from that table because the amount a server tips out is fixed, no matter if a table tips or not.
For example, it is common practise in higher end restaurants in Toronto (King Street/Yorkville) for a server to tip out 3% of the total bill to the house, 3% of the total bill to the busser/host/kitchen and 1% of the total bill to the bartenders. The server has now lost 7% of their tip automatically. That's why tipping in higher end restautants is usually set at 20%.
Example, in a higher end restauanrt, If a bill comes to $100 before tax, the bill will come to $113 after tax (13% HST). With a 20% tip, that's a $22.60 tip, but the server would be paying $7.91 in tip out (7% of total bill), meaning the server's actual tip is $14.69. Which is just shy of 15% before tax.