On PrepScholar, you recommend trying to eliminate all answers until we find the correct one. I'm working on using this technique, but sometimes I am fairly certain that the answer choice I pick is correct and feel that it will waste time to try to eliminate answers.
Should I only eliminate answer choices when I am uncertain of the right answer, or should I use this process for every question?
If you're absolutely sure that you've found the correct answer without resorting to process of elimination, there's no need to spend more time on the question. Process of elimination is for questions where you find yourself left with some uncertainty. This is because it's easier to look for one little thing that makes an answer wrong than it is to pick out the choice that's correct in every respect.
If you can answer most questions correctly on practice tests without process of elimination, that's totally fine. It's a smart strategy to have on the back burner, but time is of the essence on the ACT, so don't force yourself to use it for every question. If you come up with the solution right away, just make sure you read over the question at least one more time before filling in the corresponding bubble. Careless mistakes happen more easily when you spend less time on questions.
Source: Personal experience. I scored a 35 on the ACT last spring and didn't use process of elimination for most questions. Whenever I was uncertain of an answer, however, I employed this strategy to help me make the most logical choice.