I don't have PTCGO, nor am I able to make it to tournaents often--I'm also super impatient when it comes to waiting for online play simulators to get updated. So all of my testing has been proxy deck testing for the last 3 years.
If you are able to play both opponents and be as unbiased as possible, you can learn a lot through testing, both in how your deck works and how opposing deck X would/should/could react to things.
Thing is, the unbiased part is the hardest, but most important part of it all.