It's generally best to learn the concepts then do practice questions... but if you are going to move onto practice questions make sure that you get the MOST out of them. This includes really learning why you got a problem right or wrong, keeping an
error log, and doing them under timed conditions/through practice tests.
The only time it isn't good to work with practice problems is when you don't understand the concepts b/c then you are waisiting practice questions and not learning the underlying concepts as well.
Also, sometimes its good to learn the theory and then drill down that theory with practice questions that directly relate to that theory. So there is no definite answer, but just make sure not to just do questions to do them. You need to get everything you can out of questions when you do them. With following that advice, I don't think you can go wrong b/c practice questions can definitely be a part of your studying. You don't always have to do one thing, followed by another.
Also, here's a link to a study plan. You may already have a plan, but this plan also includes some great info on when/where to spend studying depending on where you are at:
gmat-study-plan-start-your-gmat-journey-80727.html